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

tests/libtest/lib505.c

@@ -61,6 +61,7 @@ static CURLcode test_lib505(const char *URL)
 
   /* get the file size of the local file */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   hd = stat(libtest_arg2, &file_info);
 #else
   hd = fstat(fileno(hd_src), &file_info);

tests/libtest/lib518.c

@@ -287,7 +287,7 @@ static int t518_test_rlimit(int keep_open)
 
   /* open a dummy descriptor */
 
-  t518_testfd[0] = open(DEV_NULL, O_RDONLY);
+  t518_testfd[0] = curlx_open(DEV_NULL, O_RDONLY);
   if(t518_testfd[0] < 0) {
     curl_msnprintf(strbuff, sizeof(strbuff), "opening of %s failed", DEV_NULL);
     t518_store_errmsg(strbuff, errno);

tests/libtest/lib525.c

@@ -52,6 +52,7 @@ static CURLcode test_lib525(const char *URL)
 
   /* get the file size of the local file */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   hd = stat(libtest_arg2, &file_info);
 #else
   hd = fstat(fileno(hd_src), &file_info);

tests/libtest/lib537.c

@@ -289,7 +289,7 @@ static int t537_test_rlimit(int keep_open)
 
   /* open a dummy descriptor */
 
-  t537_testfd[0] = open(DEV_NULL, O_RDONLY);
+  t537_testfd[0] = curlx_open(DEV_NULL, O_RDONLY);
   if(t537_testfd[0] < 0) {
     curl_msnprintf(strbuff, sizeof(strbuff), "opening of %s failed", DEV_NULL);
     t537_store_errmsg(strbuff, errno);

tests/libtest/lib541.c

@@ -52,6 +52,7 @@ static CURLcode test_lib541(const char *URL)
 
   /* get the file size of the local file */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   hd = stat(libtest_arg2, &file_info);
 #else
   hd = fstat(fileno(hd_src), &file_info);

tests/libtest/lib568.c

@@ -66,7 +66,7 @@ static CURLcode test_lib568(const char *URL)
   curl_free(stream_uri);
   stream_uri = NULL;
 
-  sdp = open(libtest_arg2, O_RDONLY);
+  sdp = curlx_open(libtest_arg2, O_RDONLY);
   if(sdp == -1) {
     curl_mfprintf(stderr, "can't open %s\n", libtest_arg2);
     res = TEST_ERR_MAJOR_BAD;

tests/libtest/lib572.c

@@ -84,7 +84,7 @@ static CURLcode test_lib572(const char *URL)
   stream_uri = NULL;
 
   /* PUT style GET_PARAMETERS */
-  params = open(libtest_arg2, O_RDONLY);
+  params = curlx_open(libtest_arg2, O_RDONLY);
   if(params == -1) {
     curl_mfprintf(stderr, "can't open %s\n", libtest_arg2);
     res = TEST_ERR_MAJOR_BAD;

tests/libtest/lib582.c

@@ -253,6 +253,7 @@ static CURLcode test_lib582(const char *URL)
 
   /* get the file size of the local file */
 #ifdef UNDER_CE
+  /* !checksrc! disable BANNEDFUNC 1 */
   hd = stat(libtest_arg2, &file_info);
 #else
   hd = fstat(fileno(hd_src), &file_info);

tests/server/.checksrc

@@ -3,6 +3,7 @@ allowfunc fclose
 allowfunc fopen
 allowfunc freeaddrinfo
 allowfunc getaddrinfo
+allowfunc open
 allowfunc recv
 allowfunc send
 allowfunc socket

tests/server/util.c

@@ -373,12 +373,12 @@ static void exit_signal_handler(int signum)
     (void)!write(STDERR_FILENO, msg, sizeof(msg) - 1);
   }
   else {
-    int fd;
 #ifdef _WIN32
-    fd = _open(serverlogfile, O_WRONLY|O_CREAT|O_APPEND, S_IREAD | S_IWRITE);
+#define OPENMODE S_IREAD | S_IWRITE
 #else
-    fd = open(serverlogfile, O_WRONLY|O_CREAT|O_APPEND, S_IRUSR | S_IWUSR);
+#define OPENMODE S_IRUSR | S_IWUSR
 #endif
+    int fd = open(serverlogfile, O_WRONLY | O_CREAT | O_APPEND, OPENMODE);
     if(fd != -1) {
       static const char msg[] = "exit_signal_handler: called\n";
       (void)!write(fd, msg, sizeof(msg) - 1);