/etc/rc: use mount -a instead of manual parsing

/etc/rc: use mount -a instead of manual parsing

The rc script manually parses /etc/fstab to mount all file systems.
To do that it needs /bin/sed which does not exist anymore. mount(8)
now supports the -a flag which causes it to mount all file systems
listed in /etc/fstab except for '/'. File systems marked with 'noauto'
are skipped.