Wrap mysql_real_escape_string() in a wrapper function db_escape_string()
to ease porting to other databases, and as another step to pulling more
of the database code into a central location.

This is a rebased version of a patch by elij submitted about half a year
ago.

Thanks-to: elij <elij.mx@gmail.com>
Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>