separate environment variable for XS loading debug level

Hello,

I propose to separate setting the debugging level of XS loading in
XSLoader and the mode of loading XS.  More precisely I propose a new
environment variable TEXINFO_XS_DEBUG set to 0 in the default case, that
could be set to 1 to mean something similar to current TEXINFO_XS=warn
and to 2 to mean something similar to current TEXINFO_XS=debug.

This is needed to be able to have debugging while selecting a different
loading mode than the default, such as required or requiredifenabled.

-- 
Pat