Follow the XDG Base Directory Specification

[mmicko]

Expands #4093 PR by moving code and display messages only if used with debug parameter, since yosys is fully functional without setting variables, and also history is only useful in interactive mode, and logs could spam users using it for scripting.

[nakengelhardt]

As a side effect, if -g is not used and it can't create the file, this now prints <suppressed ~1 debug messages> after the first command that you run. This is a bit misleading since it happened before the first command, should we call log_suppressed() before, e.g. in yosys_setup()? That would print it before the first prompt which is more suitable IMO:

 /----------------------------------------------------------------------------\
   <header snipped>
 \----------------------------------------------------------------------------/

 Yosys 0.37+90 (git sha1 2797d6756, clang 15.0.0 -fPIC -Os)

<suppressed ~1 debug messages>

yosys>

[nakengelhardt]

Actually, maybe driver.cc:593 (after load_plugin) would be an even better place to put it, that way it accommodates plugins using log_debug during registration as well.