Refactor: lib/services: ensure *.h declare externally hidden functions

...following the precedent of {upstart,systemd}.h. Note also that
since hardening-related commit 658fff944 [$LD -z now], it's even more
desirable to limit "symbol smog" as much as possible and symbols only
used within the library like these are trivially expendable (assuming
suitable toolchain).

As we use G_GNU_C_INTERNAL macro from glib2, we should care to require
at least that version that introduced it, i.e., 2.6 per documentation.