Add more password agent information

Add an (optional) "Id" key in the password agent .ask files. The Id is
supposed to be a simple string in "<subsystem>:<target>" form which
is used to provide more information on what the requested passphrase
is to be used for (which e.g. allows an agent to only react to cryptsetup
requests).

(v2: rebased, fixed indentation, escape name, use strappenda)