dirmngr: Do not block threads in LDAP keyserver calls.

* dirmngr/ks-engine-ldap.c: Wrap some ldap calls.

The former gpgkeys_ldap module has once been ported to dirmngr but
unfortunately the dirmngr_ldap wrapper has not been used so that we
have internal LDAP calls with these problems:

• No usable timeouts.
• On non-Windows platforms a lot of extra libs and possibly even a second copy of Libgcrypt is pulled in.
• Only one threads runs at a time.

This patch mitigates the last point.

• Signed-off-by: Werner Koch <wk@gnupg.org>