Fixed #define style.

We had a mix of styles for declaring function-like macros:

Style A:
 #define                    \
 foo()                      \
     do {                   \
         ...                \
     } while (0)

Style B:
 #define foo()              \
     do {                   \
         ...                \
     } while (0)

We had a similar number of occurences of each style:

 $ grep -rnI '^\w*(.*\\' | wc -l
 244
 $ grep -rn 'define.*(.*)' | wc -l
 239

(Those regexes aren't perfect, but a very decent approximation.)

Real examples:

 $ find src -type f | xargs sed -n '/^nxt_double_is_zero/,/^$/p'
 nxt_double_is_zero(f)                                                         \
     (fabs(f) <= FLT_EPSILON)

 $ find src -type f | xargs sed -n '/define nxt_http_field_set/,/^$/p'
 #define nxt_http_field_set(_field, _name, _value)                             \
     do {                                                                      \
         (_field)->name_length = nxt_length(_name);                            \
         (_field)->value_length = nxt_length(_value);                          \
         (_field)->name = (u_char *) _name;                                    \
         (_field)->value = (u_char *) _value;                                  \
     } while (0)

I'd like to standardize on a single style for them, and IMO,
having the identifier in the same line as #define is a better
option for the following reasons:

- Programmers are used to `#define foo() ...` (readability).
- One less line of code.
- The program for finding them is really simple (see below).

 function grep_ngx_func()
 {
     if (($# != 1)); then
         >&2 echo "Usage: ${FUNCNAME[0]} <func>";
         return 1;
     fi;

     find src -type f \
     | grep '\.[ch]$' \
     | xargs grep -l "$1" \
     | sort \
     | xargs pcregrep -Mn "(?s)^\$[\w\s*]+?^$1\(.*?^}";

     find src -type f \
     | grep '\.[ch]$' \
     | xargs grep -l "$1" \
     | sort \
     | xargs pcregrep -Mn "(?s)define $1\(.*?^$" \
     | sed -E '1s/^[^:]+:[0-9]+:/&\n\n/';
 }

 $ grep_ngx_func
 Usage: grep_ngx_func <func>

 $ grep_ngx_func nxt_http_field_set
 src/nxt_http.h:98:

 #define nxt_http_field_set(_field, _name, _value)                             \
     do {                                                                      \
         (_field)->name_length = nxt_length(_name);                            \
         (_field)->value_length = nxt_length(_value);                          \
         (_field)->name = (u_char *) _name;                                    \
         (_field)->value = (u_char *) _value;                                  \
     } while (0)

 $ grep_ngx_func nxt_sprintf
 src/nxt_sprintf.c:56:

 u_char * nxt_cdecl
 nxt_sprintf(u_char *buf, u_char *end, const char *fmt, ...)
 {
     u_char   *p;
     va_list  args;

     va_start(args, fmt);
     p = nxt_vsprintf(buf, end, fmt, args);
     va_end(args);

     return p;
 }

................
Scripted change:
................

$ find src -type f \
  | grep '\.[ch]$' \
  | xargs sed -i '/define *\\$/{N;s/ *\\\n/ /;s/        //}'

src/nxt_buf_pool.h

@@ -42,8 +42,7 @@ NXT_EXPORT void nxt_buf_pool_destroy(nxt_buf_pool_t *bp);
 
 /* There is ready free buffer. */
 
-#define                                                                       \
-nxt_buf_pool_ready(bp)                                                        \
+#define nxt_buf_pool_ready(bp)                                                \
     ((bp)->free != NULL                                                       \
      || ((bp)->current != NULL                                                \
          && (bp)->current->mem.free < (bp)->current->mem.end))
@@ -51,29 +50,25 @@ nxt_buf_pool_ready(bp)                                                        \
 
 /* A free buffer is allowed to be allocated. */
 
-#define                                                                       \
-nxt_buf_pool_obtainable(bp)                                                   \
+#define nxt_buf_pool_obtainable(bp)                                           \
     ((bp)->num < (bp)->max)
 
 
 /* There is ready free buffer or it is allowed to be allocated. */
 
-#define                                                                       \
-nxt_buf_pool_available(bp)                                                    \
+#define nxt_buf_pool_available(bp)                                            \
     (nxt_buf_pool_obtainable(bp) || nxt_buf_pool_ready(bp))
 
 
 /* Reserve allocation of "n" free buffers as they were allocated. */
 
-#define                                                                       \
-nxt_buf_pool_reserve(bp, n)                                                   \
+#define nxt_buf_pool_reserve(bp, n)                                           \
     (bp)->num += (n)
 
 
 /* Release a reservation. */
 
-#define                                                                       \
-nxt_buf_pool_release(bp, n)                                                   \
+#define nxt_buf_pool_release(bp, n)                                           \
     (bp)->num -= (n)