Removed the unsafe nxt_memchr() wrapper for memchr(3).

The casts are unnecessary, since memchr(3)'s argument is 'const void *'.
It might have been necessary in the times of K&R, where 'void *' didn't
exist.  Nowadays, it's unnecessary, and _very_ unsafe, since casts can
hide all classes of bugs by silencing most compiler warnings.

The changes from nxt_memchr() to memchr(3) were scripted:

$ find src/ -type f \
  | grep '\.[ch]$' \
  | xargs sed -i 's/nxt_memchr/memchr/'

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

src/nxt_string.h

@@ -66,10 +66,6 @@ nxt_cpymem(void *dst, const void *src, size_t length)
     (void) memmove(dst, src, length)
 
 
-#define nxt_memchr(s, c, length)                                              \
-    memchr((char *) s, c, length)
-
-
 #define nxt_strcmp(s1, s2)                                                    \
     strcmp((char *) s1, (char *) s2)