fstab-generator: Allow mount.usr without mount.usrflags, honor rw/ro

There is no need to require mount.usrflags. The original implementation
assumed that a btrfs subvolume would always be needed but that is not
applicable to systems that do not use btrfs for /usr.

Similar to using rootflags= for the default of mount.usrflags=, append
the classic 'ro' and 'rw' flags to the mount options.