Removed the unsafe nxt_memcmp() wrapper for memcmp(3).

The casts are unnecessary, since memcmp(3)'s arguments are '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_memcmp() to memcmp(3) were scripted:

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

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

src/nxt_errno.c

@@ -78,7 +78,7 @@ nxt_strerror_start(void)
 
         if (length == 0  /* HP-UX empty strings. */
             || nxt_errno == NXT_EINVAL
-            || nxt_memcmp(msg, "Unknown error", 13) == 0)
+            || memcmp(msg, "Unknown error", 13) == 0)
         {
             invalid++;
             continue;
@@ -86,8 +86,8 @@ nxt_strerror_start(void)
 
 #if (NXT_AIX)
 
-        if (nxt_memcmp(msg, "Error ", 6) == 0
-            && nxt_memcmp(msg + length - 10, " occurred.", 9) == 0)
+        if (memcmp(msg, "Error ", 6) == 0
+            && memcmp(msg + length - 10, " occurred.", 9) == 0)
         {
             invalid++;
             continue;