rc.d/sysctl: Fix error messages about unknown OIDs.

There are three situations where the sysctl script is called:

1. "start", very early
2. "lastload", near the end of rc
3. "reload", at admin request while the system is booted

Ignore unknown OIDs in situation 1 because kernel modules may not be loaded
yet and complain about them in situations 2 and 3.

PR: conf/174595
Submitted by: Olivier Smedts