GUnixMountPoint: expose options

Make the options from an /etc/fstab entry available as public API -
this can be used to support options such as

 comment=gvfs.name=Foo\040Bar

to e.g. set the name of an fstab mount in the UI to "Foo Bar".

https://bugzilla.gnome.org/show_bug.cgi?id=660536



Signed-off-by: David Zeuthen <davidz@redhat.com>