brename: rename rc.conf values via sysrc if exist
brename: rename rc.conf values via sysrc if exist

some entries in the rc.conf ASCII file are a sign of a cloud image.
we did not touch these entries when renaming a virtual machine
and for this reason the blogin could not follow the correct login instructions
fixes for #510, reported by: @aeifn.

cbsd/cbsd,cbsd/cbsd,cbsd/cbsd,cbsd/cbsd