build: avoid overriding system open and stat symbols

Replace them by `curlx_open()` and `curlx_stat()`.

To make it obvious in the source code what is being executed.

Also:
- tests/server: stop overriding `open()` for test servers.
  This is critical for the call made from the signal handler.
  For other calls, it's an option to use `curlx_open()`, but
  doesn't look important enough to do it, following the path
  taken with `fopen()`.

Follow-up to 10bac43b87 #18774
Follow-up to 20142f5d06 #18634
Follow-up to bf7375ecc5 #18503

Closes #18776

src/tool_getpass.c

@@ -30,10 +30,6 @@
 #ifndef HAVE_GETPASS_R
 /* this file is only for systems without getpass_r() */
 
-#ifdef HAVE_FCNTL_H
-#  include <fcntl.h>
-#endif
-
 #ifdef HAVE_TERMIOS_H
 #  include <termios.h>
 #elif defined(HAVE_TERMIO_H)
@@ -178,7 +174,7 @@ char *getpass_r(const char *prompt, /* prompt to display */
 {
   ssize_t nread;
   bool disabled;
-  int fd = open("/dev/tty", O_RDONLY);
+  int fd = curlx_open("/dev/tty", O_RDONLY);
   if(fd == -1)
     fd = STDIN_FILENO; /* use stdin if the tty could not be used */