trash: Correctly determine fs type for paths with symlinks

If, for example, "/home" is a symlink to "/mnt/home", and the users's $HOME
is "/home/user", then the code won't correctly recognize unix mount, because
it just walks up the path removing trailing components. Consequently, "/" is
used to determine filesystem type instead of "/mnt/home". It will cause
problems if "/mnt/home" is e.g. NFS, which is handled differently than local
filesystem.

It uses newly added g_unix_mount_for. Bump GLib dependency accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=771431