meson: allow specifying a custom "tag" for the private shared libaries

meson: allow specifying a custom "tag" for the private shared libaries

We have /usr/lib/systemd/libsystemd-{shared,core}-nnn.so. With this
path the 'nnn' part can be changed to something different. The idea
is that during a package build this will be set to the package version.

This way during in-place upgrades with the same major version both
the new and old libraries can cooexit. This should fix the issue
when systemd programs are called during package upgrades and fail
to exec because the expect different symbols in the library they
are linked to.

This should fix https://bugzilla.redhat.com/show_bug.cgi?id=1906010.