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

tests/server/.checksrc

@@ -1,4 +1,6 @@
 allowfunc accept
+allowfunc fclose
+allowfunc fopen
 allowfunc freeaddrinfo
 allowfunc getaddrinfo
 allowfunc recv

tests/server/Makefile.inc

@@ -35,6 +35,7 @@ UTILS_H =
 
 CURLX_C = \
   ../../lib/curlx/base64.c \
+  ../../lib/curlx/fopen.c \
   ../../lib/curlx/inet_pton.c \
   ../../lib/curlx/inet_ntop.c \
   ../../lib/curlx/multibyte.c \