meson: add unit installation symlinks

meson: add unit installation symlinks

This is the equivalent of $(SYSTEM_UNIT_ALIASES) and $(GENERAL_ALIASES)
in Makefile.am.

ninja-build uninstall does not remove the symlinks, see
https://github.com/mesonbuild/meson/issues/1602.
I don't consider this a blocker: after all either one installs into $DESTDIR,
where uninstallation doesn't make much sense, or into a real system, where a
successfull uninstallation would likely destroy the system.

v2:
- remove bashisms
- add various forgotten symlinks and fix service/timer/target confusions