Check arguments before eval so we don't end up with a cryptic error message.

Check arguments before eval so we don't end up with a cryptic error message.
reported by jasper@

While here: _rc_is_supported() -> _rc_not_supported()
- saves a fork
- reduces triple negation to double negation in _rc_not_supported()
- simplifie condition for rc_restart=NO
from schwarze@

ok jasper@ schwarze@