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/hiperfifo.c

@@ -383,7 +383,7 @@ static int init_fifo(struct GlobalInfo *g)
   curl_socket_t sockfd;
 
   fprintf(MSG_OUT, "Creating named pipe \"%s\"\n", fifo);
-  if(lstat(fifo, &st) == 0) {
+  if(!lstat(fifo, &st)) {
     if((st.st_mode & S_IFMT) == S_IFREG) {
       errno = EEXIST;
       perror("lstat");