Some small portability fixes (for OpenBSD)

- Provide an OpenBSD rc script
- Correctly set @plibdir on OpenBSD.
- Recognize OpenBSD as a valid @osfamily and install the rc script.
- Drop the -T flag from ln, which is non-standard.
- Retain original behaviour of ln_sfT, but without using -T.
Merge branch 'openbsd-portability'

* openbsd-portability:
  Some small portability fixes (for OpenBSD)