add ncstrwidth() that returns column width of valid prefix

[dankamongmen]

ncstrwidth() returns the column width of a utf8 string, but returns -1 if any element is invalid. depending on a given libc's unicode conformance, valid utf8 might not be recognized. ncplane_put*() will output up to the first invalid input. we ought then have an analogue, call it ehhhhh ncstrwidth_valid() (better name?):

int ncstrwidth_valid(const char* egcs, int* validbytes, *validwidth);

which will write into a non-NULL validbytes the length in bytes of the valid prefix, and write into a non-NULL validwidth the length in columns of the valid prefix. if the return value is non-negative, all input was processed (and the return value will be equal to *validwidth). if it is negative, there was an invalid character.

we could just return one or the other as the negative magnitude, but that's kinda sleazy imho (though i think we do it for ncplane_put*() actually, so maybe we ought do so here as well, for conformity?).

ncstrwidth() ought then be implemented as a static inline wrapper around this function, though that won't fly until ABI3; we'll need keep it a symbol for now.