shared/install: do not enable masked instances (#4005)

shared/install: do not enable masked instances (#4005)

When told to enable a template unit, and the DefaultInstance specified in that
unit was masked, we would do this. Such a unit cannot be started or loaded, so
reporting successful enabling is misleading and unexpected.

$ systemctl mask getty@tty1
Created symlink /etc/systemd/system/getty@tty1.service → /dev/null.
$ systemctl --root=/ enable getty@tty1
(unchanged)
Failed to enable unit, unit /etc/systemd/system/getty@tty1.service is masked.

$ systemctl --root=/ enable getty@
(before)
Created symlink /etc/systemd/system/getty.target.wants/getty@tty1.service → /usr/lib/systemd/system/getty@.service.
(now)
Failed to enable unit, unit /etc/systemd/system/getty@tty1.service is masked.

The same error is emitted for enable and preset. And an error is emmited, not a
warning, so the failure to enable DefaultInstance is treated the same as if the
instance was specified on the command line. I think that this makes most sense,
for most template units.

Fixes #2513.