tty-ask-password-agent: log when starting a query on the console

tty-ask-password-agent: log when starting a query on the console

When looking at logs from a boot with an encrypted device, I see
(with unrelevant messages snipped):
[    2.751692] systemd[1]: Started Dispatch Password Requests to Console.
[    7.929199] systemd-cryptsetup[258]: Set cipher aes, mode xts-plain64, key size 512 bits for device /dev/disk/by-uuid/2d9b648a-15b1-4204-988b-ec085089f8ce.
[    9.499483] systemd[1]: Finished Cryptography Setup for luks-2d9b648a-15b1-4204-988b-ec085089f8ce.

There is a hug gap in timing without any explanatory message. If I didn't type
in the password, there would be no way to figure out why things blocked from
this log, so let's log something to the log too.