/etc/mtab can be a link to a file in /proc. If it is, we should not
attempt to update /etc/mtab.
The original test used "! -w" as part of the test. This does not
work since everything is writeable by root.
Thanks to Robin Johnson for the suggestion of using readlink -f and the
regular expression.
Reported-By: junkmailnotread@yahoo.com
X-Gentoo-Bug: 370037
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=370037