bug#81135: use of quotearg in the "C" locale

[Paul Eggert]

I attempted to address the problem by having the USE_C_LOCALE code in 
Gnulib treat unassigned character positions as printable, in this Gnulib 
patch:

https://cgit.git.savannah.gnu.org/cgit/gnulib.git/commit/?id=d9723b157578980c0c37782408a89e381393f032
The idea is that this should work well enough in today's likely locales.