[PATCH][BZ 19108] paths: use /proc/mounts instead of mtab

[Jason A. Donenfeld]

I'm not really part of their camp, but the systemd people write this
in their release notes:

https://github.com/systemd/systemd/blob/master/NEWS

> Any reference to /etc/mtab has been dropped from
> systemd. The file has been obsolete since a while, but
> systemd refused to work on systems where it was incorrectly
> set up (it should be a symlink or non-existent). Please make
> sure to update to util-linux 2.27.1 or newer in conjunction
> with this systemd release, which also drops any reference to
> /etc/mtab. If you maintain a distribution make sure that no
> software you package still references it, as this is a
> likely source of bugs. There's also a glibc bug pending,
> asking for removal of any reference to this obsolete file:
> https://sourceware.org/bugzilla/show_bug.cgi?id=19108