Implement PGP key search and import
Add two new static methods, key_search() and key_import(), to our
growing list of signing code.
If we come across a key we do not have, attempt to look it up remotely
and ask the user if they wish to import said key. If they do, flag the
validation process as a potential 'retry', meaning it might succeed the
next time it is ran.
These depend on you having a 'keyserver hkp://foo.example.com' line in
your gpg.conf file in your gnupg home directory to function.
Signed-off-by: Dan McGee <dan@archlinux.org>