agent: Add optional ecdh parameter arg to KEYTOCARD.

* agent/command.c (KEYTOCARD_TIMESTAMP_FORMAT): Remove and use format
string direct.
(cmd_keytocard): Change timestamp to an u64 and use the new u64 parser
functions.  Use split_fields.  Add ecdh parameter stuff.  Take the
default timestamp from the keyfile.
* agent/findkey.c (agent_key_from_file): Add arg timestamp and set it.
Adjust all callers.

This is backport from 2.4 but only the agent part. We consider it
more relibale if we also pass the ECDH parameters along in 2.2.
Adding the timestamp stuff should not harm either. Because we now
have the u64 time string parser, we use them here.

See-commit: c03ba92576e34f791430ab1c68814ff16c81407b
See-commit: c795be79c14fac01b984bdc2e2041d2141f27612
This patch is somewhat related to:

• GnuPG-bug-id: T6378, T5538