utf8.c(): Default to allow problematic code points

utf8.c(): Default to allow problematic code points

Surrogates, non-character code points, and code points that aren't in Unicode
are now allowed by default, instead of having to specify a flag to allow them.
(Most code did specify those flags anyway.)

This affects uvuni_to_utf8_flags(), utf8n_to_uvuni() and various routines that
are specialized interfaces to them.

Now there is a new set of flags to disallow those code points.  Further, all 66
of the non-character code points are known about and handled consistently,
instead of just U+FFFF.

Code that requires these code points to be forbidden will have to change to use
the new flags.  I have looked at all the (few) instances in CPAN where these
routines are used, and the only one I found that appears to have need to do
this, Encode, has already been patched to accommodate this change.  Of course,
I may have overlooked some subtleties.