Automatically learn keys from smartcards

* src/keycache.cpp (do_populate_smartcards)
(gpgagent_transact, gpgsm_learn): New.

This checks if there are unknown keys on a smartcard
by the keygrip. If keys are not found then gpgsm --learn-card
is called.

The idea behind this is to have a plug and play mechanism
for Smartcards.

• GnuPG-Bug-Id: T4877