Add support for /conf/T/M/remount_optional.

Add support for /conf/T/M/remount_optional.

The rc.initdiskless functionality is used by NanoBSD to allow configuration
files to live on a separate configuration slice, which acts as NVRAM, whilst
the system image is mounted read-only.

Normally, if the remount command fails during boot, this is regarded as
a fatal error. If /conf/T/M/remount_optional is present, this error is
non-fatal. If the file is not present, the default behaviour is unchanged.

This is very useful for people building live CD images using FreeBSD,
where the NVRAM lives somewhere completely differently from the system image,
and may be present on removable media which is not present during the
initial boot.