* gnu/build/file-systems.scm (MS_REMOUNT): New constant.
(mount-file-system): Add 'flags' local variable. When FLAGS has
MS_BIND & MS_RDONLY, call 'mount' with MS_REMOUNT.
* gnu/services/base.scm (file-system-service) <start>: Likewise.
* gnu/build/file-systems.scm (mount-file-system): When OPTIONS is false,
write "rw" to /etc/mtab. This fixes a bug whereby 'remove-from-mtab'
in (guix syscalls) would otherwise raise a 'match-error' because of
the missing field. This affected file systems mounted from the
initrd, such as devtmpfs, which has options == #f.