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.