gunixmounts: Speed up mtab processing with libmount
gunixmounts: Speed up mtab processing with libmount

libmnt_context is useless. It contains cache which is useful for searching,
but it isn't used in our case. Let's use mnt_context_parse_mtab instead
directly and the mtab processing will be faster.

https://bugzilla.gnome.org/show_bug.cgi?id=781867

endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib