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/perl/nxt_perl_psgi.c

@@ -765,7 +765,7 @@ nxt_perl_psgi_result_status(PerlInterpreter *my_perl, SV *result)
 
     status.start = (u_char *) SvPV(*sv_status, status.length);
 
-    space = nxt_memchr(status.start, ' ', status.length);
+    space = memchr(status.start, ' ', status.length);
     if (space != NULL) {
         status.length = space - status.start;
     }