Allow to provide the db password inside a file instead of using

Allow to provide the db password inside a file instead of using
the command line, so that it isn't exposed in the process list.

Sadly, use of /dev/stdin or <(process) doesn't work in Linux,
since they are shown as symlinks to pipe:[12345678] and php
dereferences all of them. It has to be a real file.
However, such constructs work in Solaris, where they are
presented as character devices.