* protect.c (agent_private_key_type): New.
* agent.h: Add PRIVATE_KEY_ enums.
* findkey.c (agent_key_from_file): Use it to decide whether we
have to unprotect a key.
(unprotect): Cache the passphrase.
* findkey.c (agent_key_from_file,agent_key_available): The key
files do now require a ".key" suffix to make a script's life
easier.
* genkey.c (store_key): Ditto.