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)
Internally we store all time values in usec_t, however parse_usec()
actually was used mostly to parse values in seconds (unless explicit
units were specified to define a different unit). Hence, be clear about
this and name the function about what we pass into it, not what we get
out of it.
