tmpfiles.eclass: Support using on non-Linux systems

tmpfiles.eclass: Support using on non-Linux systems

Fix the eclass code to remove the misguided Linux conditionals.
The whole purpose of the eclass was to avoid having to implement
fallback logic for systems not having service manager tmpfiles.d
support. Making it conditional to Linux implied that for non-Linux
systems (Prefix, FreeBSD) we would have to implement explicit fallback
to create the necessary directories.

While systemd (and therefore systemd-tmpfilesd) is indeed
Linux-specific, the opentmpfiles implementation should be pretty
portable and there is no reason to restrict it to Linux only, or to
prevent using it on non-Linux OpenRC (and non-OpenRC) systems.