gio: Add g_unix_mount_get_options

GVfsUDisks2VolumeMonitor handles x-gvfs-hide/x-gvfs-show mount options
used to overwrite our heuristics whether the mount should be shown, or
hidden. Unfortunately, it works currently only for mounts with
corresponding fstab entries, because the options are read over
g_unix_mount_point_get_options. Let's introduce g_unix_mount_get_options
to allow reading of the options for all sort of mounts (e.g. created
over pam_mount, or manually mounted).

(Minor fixes to the documentation by Philip Withnall
<withnall@endlessm.com>.)

https://bugzilla.gnome.org/show_bug.cgi?id=668132