logind: add code for UTMP wall messages

logind: add code for UTMP wall messages

Add a timer to print UTMP wall messages so that it repeatedly informs users
about a scheduled shutdown:

 * every 1 minute with less than 10 minutes to go
 * every 15 minutes with less than 60 minutes to go
 * every 30 minutes with less than 180 minutes (3 hours) to go
 * every 60 minutes if more than that to go

This functionality only active if the .EnableWallMessages DBus property
is set to true. Also, a custom string can be added to the wall message,
set through the WallMessagePrefix property.