cmake GUI cleanup

mark path-related variables (AIO_LIBRARY, ODBC_LIBRARY, ODBC_INCLUDE_DIR,
Thrift_LIBS, Thrift_INCLUDE_DIRS, CRYPTO_LIBRARY, OPENSSL_LIBRARIES,
OPENSSL_ROOT_DIR, OPENSSL_INCLUDE_DIR) as advanced - paths are
automatically discovered by cmake.

mark few choice variables (ENABLED_LOCAL_INFILE, WITHOUT_SERVER,
DISABLE_SHARED) as not advanced - they are user choices, not automatically
configured values.

remove unused BACKUP_TEST variable.
MDEV-6248 GUI-friendly cmake options to enable/disable plugins

* Introduce a set of PLUGIN_xxx cmake options with values
  NO, STATIC, DYNAMIC, AUTO, YES (abort if plugin is not compiled)
* Deprecate redundant and ambiguous WITH_xxx, WITH_PLUGIN_xxx,
  WITH_xxx_STORAGE_ENGINE, WITHOUT_xxx, WITHOUT_PLUGIN_xxx,
  WITHOUT_xxx_STORAGE_ENGINE
* Actually check whether a plugin is disabled (DISABLED keyword was
  always present, but it was ignored until now).
* Support conditionally disabled plugins - keyword ONLY_IF
* Use ONLY_IF for conditionally skipping plugins, instead of
  doing MYSQL_ADD_PLUGIN conditionally as before. Because if
  MYSQL_ADD_PLUGIN isn't done at all, PLUGIN_xxx=YES cannot work.