systemctl: Support all unit types in the directives.

The Alias and WantedBy directives can accept all valid unit types when
using the systemctl from systemd. And since the systemctl script should
match the behavior of systemd as much as possible, add the current set
of unit types listed at
http://www.freedesktop.org/software/systemd/man/systemd.unit.html
to the Alias and WantedBy directives.

The deficiency was exposed when trying to use:
    Alias=default.target
in a foo.target. No symlink was created by running
"systemctl enable foo.target" during the package's postinst.

Signed-off-by: Randy Witt <rewitt@declaratino.com>