service: sysv - properly handle init scripts with .sh suffix

Dropping the distribution specific #ifdefs in
88516c0c952b9502e8ef1d6a1481af61b0fb422d broke the .sh suffix stripping
since we now always used the else clause of the rc. check.

We eventually want to drop the rc. prefix stripping, but for now we
assume that no sysv init script uses both an rc. prefix and .sh suffix,
so make the check for the .sh suffix and rc. prefix mutually exclusive.