install: allow specifiers in WantedBy/RequiredBy/Alias

install: allow specifiers in WantedBy/RequiredBy/Alias

This allows one templated unit to refer to another templated unit
at installation time.

Examples:

> grep WantedBy ~/.config/systemd/user/mpop@.timer
WantedBy=services@%i.target

> srv disable mpop@iit.timer
rm '/home/alxchk/.config/systemd/user/services@iit.target.wants/mpop@iit.timer'
> srv enable mpop@iit.timer
ln -s '/home/alxchk/.config/systemd/user/mpop@.timer' '/home/alxchk/.config/systemd/user/services@iit.target.wants/mpop@iit.timer'

Based-on-patch-by: Oleksii Shevchuk <alxchk@gmail.com>