when wcsrtombs stopped due to hitting zero remaining space in the
output buffer, it was wrongly clearing the position pointer as if it
had completed the conversion successfully.
this commit rearranges the code somewhat to make a clear separation
between the cases of ending due to running out of output buffer space,
and ending due to reaching the end of input or an illegal sequence in
the input. the new branches have been arranged with the hope of
optimizing more common cases, too.
negative values of wchar_t need to be treated in the non-ASCII case so
that they can properly generate EILSEQ rather than getting truncated
to 8bit values and stored in the output.
to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.
