XS module not loaded and erroring out in the default case

Hello,

First, I finished replacing individual functions overriding by full
interface modules.  There is one downsize, many Perl modules are now
divided in two files, but overall I think that the clarification of what
is and what is not in XS is worth it.  Individual functions overriding
remain for MiscXS and ConfigXS.

I think that the current situation regarding erroring out in case
some XS interfaces are not loaded in the default case, with TEXINFO_XS
unset is not the best.  Indeed, for XS interfaces requiring the use of
the XS/C parser, it is erroneous to have only some XS interface files
loaded.  The errors are diverse, some with visible messages
(document/converter not found), or segfaults, some with silent incorrect
output.  None of those errors should be seen by users.

I think that instead texi2any should abort with a message telling that
some XS interface object code could not be loaded.  In practice, this
would mean making the TEXINFO_XS requiredifenabled behaviour the default
for XS interfaces requiring the use of the XS/C parser.  The Paragraph
XS and Misc XS interfaces could still silently fail to be loaded in the
default case.

What do you think?

-- 
Pat