Add the _2 interfaces plus a comment in "libimagequant.h".
Document _2 functions. Delete strip-c99.pl.

This adds more documentation (not as good as "README.md" but not
nothing) for the _2 functions created for the Perl interface.

strip-c99.pl is OK on the branch "new-interface-again", so use that
one to strip these out.

Stripping C99 out can wait until we have finished the main parts of the
interface.