Add command line option to specify path to user.cfg

[Dunedan]

For testing lobby related stuff I often have to run Pyrogenesis with different configuration options. While that's already possible using the -conf command line option, it becomes messy when running multiple instances of Pyrogenesis in parallel, as they'll all write to the same user.cfg file. Therefore, it'd be great to be able to specify a non-default location of the user.cfg to read and write to as command line parameter.

[Ralph Sennhauser]

Setting HOME or better yet XDG_CONFIG_HOME doesn't work for you?

[Dunedan]

$HOME is too broad and would for example also result in a copy of cached data ($HOME/.cache/0ad) per instance.

$XDG_CONFIG_HOME would work, but that's probably limited to Linux and doesn't work for Windows and MacOS.

[Stan]

Well on Windows you can only run one instance of the game using the same exe. So you need N installs. And if you want them to be independant you just use the -writableRoot command line argument.