regcomp.c: Fix comment
regen/regcharclass.pl: Improve the generated code

This is a small improvement when a consecutive group of U8 code points
begins at 0 or ends at 255.  These end points are physically impossible
of being exceeded, so there is no need to test for that end of the
range.  In several places this causes a mask operation to not be
generated.
Wrap various pod uses of NUL with C<>

This makes the uses of this consistent in our pods.  Also changed one
use of the word 'buffer' into 'string', the latter being more
appropriate.