ask-password: when querying for a password, try to read from credential store first

ask-password: when querying for a password, try to read from credential store first

This adds generic support for the SetCredential=/LoadCredential= logic
to our password querying infrastructure: if a password is requested by a
program that has a credential store configured via
$CREDENTIALS_DIRECTORY we'll look in it for a password.

The "systemd-ask-password" tool is updated with an option to specify the
credential to look for.