rectify the configuration file location #1066
Description
Probably a not well-known feature of bap, but we do have the configuration file. However, its location is quite inconvenient (it is $etc/bap, which is if you have an opam installation $(opam var bap:etc) or if you don't have bap installed but have opam, then (the same location) $(opam var etc)/bap). Not only it doesn't make a lot of sense to have separate configuration for each opam switch, but it is also quite hard to find it (I myself have to look into the source code every time to figure out where it is).
What we need, is to store the main configuration file in the XDG prescribed location, i.e., somewhere in the home folder, most likely it would be ~/.config/bap. We can also implement a hierarchy of configuration files with the site file and local file, but probably there is no need to overcompilcate things, we can always add this feature later, when we will need it.