build: avoid overriding system symbols for fopen functions

By introducing wrappers for them in the curlx namespace:
`curlx_fopen()`, `curlx_fdopen()`, `curlx_fclose()`.

The undefine/redefine/`(function)()` methods broke on systems
implementing these functions as macros. E.g. AIX 32-bit's `fopen()`.

Also:
- rename `lib/fopen.*` to `lib/curl_fopen.*` (for `Curl_fopen()`)
  to make room for the newly added `curlx/fopen.h`.
- curlx: move file-related functions from `multibyte.c` to `fopen.c`.
- tests/server: stop using the curl-specific `fopen()` implementation
  on Windows. Unicode isn't used by runtests, and it isn't critical to
  run tests on longs path. It can be re-enabled if this becomes
  necessary, or if the wrapper receives a feature that's critical for
  test servers.

Reported-by: Andrew Kirillov
Bug: https://github.com/curl/curl/issues/18510#issuecomment-3274393640

Follow-up to bf7375ecc5 #18503
Follow-up to 9863599d69 #18502
Follow-up to 3bb5e58c10 #17827

Closes #18634

scripts/checksrc.pl

@@ -88,6 +88,9 @@ my %banfunc = (
     "send" => 1,
     "socket" => 1,
     "socketpair" => 1,
+    "fclose" => 1,
+    "fdopen" => 1,
+    "fopen" => 1,
     );
 
 my %warnings_extended = (
@@ -919,8 +922,8 @@ sub scanfile {
         }
 
         # scan for use of non-binary fopen without the macro
-        if($l =~ /^(.*\W)fopen\s*\([^,]*, *\"([^"]*)/) {
-            my $mode = $2;
+        if($l =~ /^(.*\W)(curlx_fopen|CURLX_FOPEN_LOW)\s*\([^,]*, *\"([^"]*)/) {
+            my $mode = $3;
             if($mode !~ /b/) {
                 checkwarn("FOPENMODE",
                           $line, length($1), $file, $ol,