locale support patch for [fs]sscanf function

[CdeMills]

Hello,

herewith enclosed a patch proposal against 3.6.0 implementing locales
support in [fs]sscanf. There is no API change; the "C" argument is re-used
to pass the locale value. And "C" is BTW a valid locale name.

Mechanism:
- class octave_base_stream gets a new virtual member, imbue, which does
nothing
- class octave_stream gets the real implementation, its a mere proxy to
underlying std::[io]stream::imbue
- function ffscanf and scanf detect it the right argument is a stream, and
call stream::imbue in this case. For the scanf implementation, the original
locale is not preserved. For the ffscanf, it goes as:
           {
              // Use its val as the new locale setting. Keep old val
              // for restoring afterwards.
              std::locale oldloc = 
                os.imbue (std::locale (args(2).string_value ().c_str ()));
              retval = os.oscanf (args(1), who);
              os.imbue (oldloc);
            }
This way, locale setting is preserved. This is a bit overkill in case of
repeated accesses, but this avoids the need to introduce a new interface to
set/reset the locale.

Advices ? Comment ?

Pascal
http://octave.1599824.n4.nabble.com/file/n4303471/sscanf-locale.pat
sscanf-locale.pat 

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