[debian-mysql] Bug#792918: Bug#792918: mariadb-10.0: upgrade fails when /var/lib/mysql is a soft link

[Otto Kekäläinen]

Thank you for your PR at https://github.com/ottok/mariadb-10.0/pull/25

I will merge it and after that it will automatically be available in
Debian unstable and eventually in new releases of Debian and Ubuntu.
Due to Debian and Ubuntu policies it is difficult to backport these
kind of changes and get bugfixes into stable releases.

Note: In the postinstall file there is a line
```mysql_upgradedir=/var/lib/mysql-upgrade``` defined but it is not
used anywhere. I wonder if the original feature is used at all.

The MariaDB packaging is inherited from
https://anonscm.debian.org/cgit/pkg-mysql/mysql-5.6.git/tree/debian/mysql-server-5.6.preinst?id=refs/heads/master#n140
and non-use of it from
https://anonscm.debian.org/cgit/pkg-mysql/mysql-5.6.git/tree/debian/mysql-server-5.6.postinst?id=refs/heads/master

I don't see any particular spot where the mysql-upgrade directory is
used, thus I don't know where it would be safe to clean it up.

Anyway the line in question works if the symbolic link to the mysql
datadir is relative and fails it if is absolute. Your '-T' fixes the
cases where the datadir link is absolute and dpkg will at least not
fail on upgrades in those cases in the future.