Add support for displacing bmonrc into $XDG_CONFIG_HOME/bmon/

[lelutin]

Hello,

bmon is one of the few apps I use that still require its config file to be placed directly at the top of my home directory. It would be nice to make it so bmon would search for its rc file under $XDG_CONFIG_HOME/bmon/bmonrc in addition to the current path ~/.bmonrc -- also if the env var $XDG_CONFIG_HOME is not defined, then the first path should default to ~/.config/bmon/bmonrc.

There's an interesting list of all apps that currently support this: https://wiki.archlinux.org/title/XDG_Base_Directory

cheers!

p.s.: if you would rather have someone else implement this and send a PR, I can try. C is not a language that I master, but for such a small change I hopefully won't cause much of a blunder