tests: move GSS-API dynamic stub into debug-mode libcurl

Replace the `libstubgss.so`-based overload solution with one built into
libcurl at compile-time.

The previous, `LD_PRELOAD`-based, solution was non-portable, allowlisted
for Linux, BSD and Solaris. It also required non-debug builds, which
turned out to be an accidental condition:
7d342c723c. It also required a curl tool
built against a shared libcurl. Detecting this condition wasn't always
accurate, e.g. with certain cmake configurations.

The overload solution also didn't work on macOS, though it theoretically
should have:
- #17653
- #2394

Experiments on making the overload solution work in more envs:
- #17759
  That revealed that it also did not work on NetBSD, in CI.

The replacement solution is overloading the necessary GSS-API functions
for test 2056 and 2057 at compile time. It requires a debug-enabled curl
build (due to its insecure nature).

This makes these tests run on all platforms. Including most GSS jobs in
CI, that are running tests. (the exception is old-linux, non-debug jobs,
where it felt overkill to enable debug for this.)

The refactored GSS stub code needs to overload less than before because
it's free to use the official GSS API. (This didn't work with
the overload solution on Alpine for example). It can also use libcurl
functions, allowing to replace `snprintf()` with `msnprintf()`.

OS/400 is also overloading GSS API functions. I haven't tested how this
works after this PR. In theory it should, because this PR doesn't rely
on preprocessor overrides.

Note that for future GSS tests, it may be necessary to stub these GSS
API functions: `gss_inquire_context()`, `gss_unwrap()`, `gss_wrap()`.
They are on codepaths not (yet) touched by tests.

Also:
- stub-gss: check for token buffer overrun.
- stub-gss: replace size macros with `sizeof()`.
- GHA: enable debug for some jobs with GSS.
- GHA/linux: ignore results for 2056 and 2057 in the valgrind job.
  They leak the same way as seen with 2077 and 2078.
  Ref: 7020ba7979 #17462
  Ref: 146759716c #14430
- GHA/linux: fix to ignore `gss_import_name()` leaks in valgrind builds.
  only.
- lib/vauth/krb5_gssapi: reduce variable scope.
- lib/vauth/spnego_gssapi: reduce variable scope.
- tests/libtest: drop code and build logic dealing with `libstubgss`.
- runtests:
  - drop `ld_preload` feature.
  - drop special handling of `LD_PRELOAD` env in tests.
  - drop logic dealing with shared curl tool detection.
  - drop `LD_PRELOAD` envs from tests.

Follow-up to 56d949d31a #1687

Closes #17752

lib/curl_gssapi.h

@@ -32,17 +32,20 @@ extern gss_OID_desc Curl_spnego_mech_oid;
 extern gss_OID_desc Curl_krb5_mech_oid;
 
 /* Common method for using GSS-API */
-OM_uint32 Curl_gss_init_sec_context(
-    struct Curl_easy *data,
-    OM_uint32 *minor_status,
-    gss_ctx_id_t *context,
-    gss_name_t target_name,
-    gss_OID mech_type,
-    gss_channel_bindings_t input_chan_bindings,
-    gss_buffer_t input_token,
-    gss_buffer_t output_token,
-    const bool mutual_auth,
-    OM_uint32 *ret_flags);
+OM_uint32 Curl_gss_init_sec_context(struct Curl_easy *data,
+                                    OM_uint32 *minor_status,
+                                    gss_ctx_id_t *context,
+                                    gss_name_t target_name,
+                                    gss_OID mech_type,
+                                    gss_channel_bindings_t input_chan_bindings,
+                                    gss_buffer_t input_token,
+                                    gss_buffer_t output_token,
+                                    const bool mutual_auth,
+                                    OM_uint32 *ret_flags);
+
+OM_uint32 Curl_gss_delete_sec_context(OM_uint32 *min,
+                                      gss_ctx_id_t *context_handle,
+                                      gss_buffer_t output_token);
 
 /* Helper to log a GSS-API error status */
 void Curl_gss_log_error(struct Curl_easy *data, const char *prefix,