* src/common.cpp (get_gpg4win_dir): Rewrite to get rid of the the need
for Registry keys.  Also tell gpgme about the installation directory
on the first call.  Return a const string and change all callers
accordingly.
(get_gpgme_w32_inst_dir): Remove.
* src/gpgoladdin.cpp (GpgolAddin::GpgolAddin): Do not use
get_gpgme_w32_inst_dir but simply call get_gpg4win_dir for its
side-effect.
* src/wks-helper.cpp (get_wks_client_path): Change the order on how to
find gpg-wks-client.  A GnuPG subdir has now priority.