dirmngr: Use the LDAP wrapper process also for Windows.

* dirmngr/ldap-wrapper.c: Revamp module to make use of es_poll for
portability.
* configure.ac: Always use the ldap wrapper.

Since the migration from GNU Pth to nPth the ldap wrapper never worked
reliable on Windows. Our long term use of the old Window CE wrapper
thing didn't fixed this either. The new code uses the portable
es_poll function and thus code which is tested at several other
places. It Should(tm) fix the Windows issues.

• GnuPG-bug-id: T3937
• Signed-off-by: Werner Koch <wk@gnupg.org>