Fix get_mounts_timestamp() to not use a stat'ed mtime for /proc/ files.
Instead, use mount_poller_time if /proc/ watch is running, or otherwise
return a new generated timestamp to always assume mounts-changed, which
is safer than previous behaviour of always assuming mounts-not-changed
(as mtime never changes for /proc/ files when queried from the same
process).

We say it's safer because allows caches depending on:

g_unix_mounts_get(&time_read)
g_unix_mounts_changed_since()

to drop possibly outdated/duplicated values, as that was the case for the
GIO mounts cache used in gio/glocalfile.c which provides mount info for
g_file_query_filesystem_info() call, as described in below referenced bug.

This fix complements related commit bd9e266e

https://bugzilla.gnome.org/show_bug.cgi?id=787731