Tighten the ranges in wcrtomb(3).

Tighten the ranges in wcrtomb(3).

By definition, the range of valid Unicode code points is the union of
U+0000..U+D7FF and U+E000..U+10FFFF (see Unicode 8.0.0, chapter 3.9).

In UTF-16, the encoded values that would represent U+D800..U+DFFF are
used for surrogate pairs. UTF-8 has no concept of surrogate pairs;
attempting to treat them as regular code points violates the standard
and makes no sense besides.

ok stsp@