w32: Try to locate gpg in the gpgme installation dir.

* src/w32-util.c (my_hmodule): New.
(wchar_to_utf8): New.
(DllMain): New.
(_gpgme_get_inst_dir): New.
(find_program_in_dir): New.
(find_program_in_inst_dir): Add arg INST_DIR.
(_gpgme_get_gpg_path): Get inst_dir before acquiring the lock.
(_gpgme_get_gpgconf_path): Ditto.
(_gpgme_get_g13_path): Ditto.
(_gpgme_get_w32spawn_path): Ditto.