core,w32: More robust detection of GnuPG Desktop dir layout
* src/w32-util.c (INST_TYPE_GPG4WIN, INST_TYPE_GPGDESK): New to replace the numeric constants. (find_version_file): New. (_gpgme_get_gpgconf_path): Guess a default for inst_type.
- GnuPG-bug-id: T6362