cdefs.h: ignore known rcs id macros

This helps to retain history, reduces FreeBSD porting diffs
slightly and won't introduce binary changes.  Adjust the
porting doc accordingly.

Discussed-with: swildner
Taken-from:     FreeBSD
kernel/ath: Add missing __printflike().
Remove the old if_ixgbe.ko via 'make upgrade'.
doc: how to port devfs_cdevpriv(9)
doc: an equivalent of devfs_cdevpriv(9) is not availiable

Thanks to alexh for pointing that out.  I would like to see
more of these things in the porting doc so that they don't
become a general tripwire.