agent: Add checkpin inquiry for pinentry
* agent/call-pinentry.c (inq_cb): Handle checkpin inquiry. (setup_enforced_constraints): New. (agent_get_passphrase): Call setup_enforced_constraints if new passphrase is requested.
This implements the gpg-agent side for checking whether a new passphrase
entered by the user in pinentry satisfies the passphrase constraints.
Performing a checkpin inquiry is only allowed if the passphrase
constraints are enforced. setup_enforced_constraints sends necessary
options and translated strings to pinentry.