Add support for picking up the proverc file from environment

[Corion]

This adds support to find the proverc in other locations that are
usually preferred over the home directory:

$ENV{XDG_CONFIG_HOME}, which is where the "new" place for
user config is under OpenDesktop

$ENV{HOME}, as not every OS/shell sets ~

$ENV{USERPROFILE} for Windows

~ for Unix(ish) OSes

Maybe in a second step it makes sense to optionally pass the
%ENV into ->process_args()

[Leont]

The removal of that glob is definitely a bug, that '~' will no longer work. And actually, glob already handles HOME and USERPROFILE.

Adding XDG_CONFIG_HOME support makes enough sense to me, the rest doesn't