Protect __KERNEL_RCSID. It isn't available when compile rumpkernels under e.g. Linux.

Merge the [pgoyette-compat] branch

Add missing RCSIDs.

Unconditionally include <unistd.h>, as we use ftruncate() unconditionally
too.
Reported (with patch) by Antonio Huete Jimenez in PR 53153, slightly
modified by me.

rename rumpcomp_user.* -> shmif_user.*