systemctl: print next timer trigger time with the status verb (#6242)

systemctl: print next timer trigger time with the status verb (#6242)

It is useful to know when a timer will trigger next when looking at a
timer status message so calculate and print that information.

Closes #5738.

Example output:
$ systemctl status dnf-makecache.timer
● dnf-makecache.timer - dnf makecache timer
   Loaded: loaded (/usr/lib/systemd/system/dnf-makecache.timer; enabled; vendor preset: enabled)
   Active: active (waiting) since Tue 2017-07-04 17:24:02 EDT; 24min ago
  Trigger: Tue 2017-07-04 18:15:56 EDT; 27min left