Make auto import from S/MIME keyserver default off

* src/common_indep.h (opt.search_smime_servers): New.
* src/main.c (read_options): Read it.
* src/keycache.cpp (do_locate): Respect it.

This also does not do an external search if there
was an OpenPGP certificate found for the recipient and
prefer_smime is not set.