openbsd_rcctl: improve performance of available() and missing()

Previously, these 2 functions were very expensive because they needed to
parse the output of the full services status. I modified rcctl(8) in
OpenBSD to return ENOENT when a service does not exist which is
instantaneous.

`time salt-call service.available sshd`
before
    0m4.82s real     0m0.42s user     0m3.05s system
after
    0m1.38s real     0m0.40s user     0m0.78s system