This is an extraction of ArgsManager related functions from util/system
into their own common file.
Config file related functions are moved to common/configfile.cpp.
The background of this commit is an ongoing effort to decouple the
libbitcoinkernel library from the ArgsManager. The ArgsManager belongs
into the common library, since the kernel library should not depend on
it. See doc/design/libraries.md for more information on this rationale.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/27419 | core#27419]]
Note that due to missing backports, I had to make `CheckValid` and `InterpretOption` non-static and add them to args.h so that they are available both for args.cpp and configfile.cpp. These two static functions will go away when we backport [[https://github.com/bitcoin/bitcoin/pull/22766 | core#22766]]