patch 9.1.1076: vim_strnchr() is strange and unnecessary

Problem:  vim_strnchr() is strange and unnecessary (after v9.1.1009)
Solution: Remove vim_strnchr() and use memchr() instead.  Also remove a
          comment referencing an #if that is no longer present.

vim_strnchr() is strange in several ways:
- It's named like vim_strchr(), but unlike vim_strchr() it doesn't
  support finding a multibyte char.
- Its logic is similar to vim_strbyte(), but unlike vim_strbyte() it
  uses char instead of char_u.
- It takes a pointer as its size argument, which isn't convenient for
  all its callers.
- It allows embedded NULs, unlike other "strn*" functions which stop
  when encountering a NUL byte.

In comparison, memchr() also allows embedded NULs, and it converts bytes
in the string to (unsigned char).

closes: #16579

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/strings.c

@@ -674,22 +674,6 @@ vim_strchr(char_u *string, int c)
     return NULL;
 }
 
-// Sized version of strchr that can handle embedded NULs.
-// Adjusts n to the new size.
-    char *
-vim_strnchr(const char *p, size_t *n, int c)
-{
-    while (*n > 0)
-    {
-	if (*p == c)
-	    return (char *)p;
-	p++;
-	(*n)--;
-    }
-
-    return NULL;
-}
-
 /*
  * Version of strchr() that only works for bytes and handles unsigned char
  * strings with characters above 128 correctly. It also doesn't return a
@@ -3558,8 +3542,6 @@ vim_vsnprintf_typval(
 			str_arg_l = 0;
 		    else
 		    {
-			// Don't put the #if inside memchr(), it can be a
-			// macro.
 			// memchr on HP does not like n > 2^31  !!!
 			char *q = memchr(str_arg, '\0',
 				  precision <= (size_t)0x7fffffffL ? precision