tidy-up: drop stray comparisons with literal zero

Drop from:
- strcmp, strcmpi, strncmp, memcmp, lstat, getrlimit, setrlimit, fseek,
  fstat
- autotools detection snippets.
- smooth-gtk-thread: simplify `!var != 0` expression.

Closes #21947

docs/examples/fileupload.c

@@ -64,7 +64,7 @@ int main(void)
   }
 
   /* to get the file size */
-  if(fstat(fileno(fd), &file_info) != 0) {
+  if(fstat(fileno(fd), &file_info)) {
     fclose(fd);
     curl_global_cleanup();
     return 1; /* cannot continue */