regcomp.c: Extract out functionality into a function

This is in preparation for it being called from a 2nd place.  The code
was merely moved and outdented, and comments moved within the function
and added to.
Allow \N{named seq} in qr/[...]/

This commit changes the regex handler to properly match in many
instances a \N{named sequence} in a bracketed character class.

A named sequence is one which consists of a string of multiple
characters but given one name.  Unicode has hundreds of them, like LATIN
CAPITAL LETTER A WITH MACRON AND GRAVE.  These are encoded by Unicode
when there is some user community that thinks of the conglomeration as a
single unit, but there was no prior standard that had it so, and it is
possible to encode it in Unicode using other means, typically a sequence
of a base character followed by some combining marks.  (If there had not
been such a prior standard, 8859-1, things like LATIN CAPITAL LETTER A
WITH GRAVE would have been put into Unicode this way too.)  If they did
not do it this way, they would run out of availble code points much
sooner.

Not having these as single characters adds a burden to the programmer
having to deal with them.  Hiding this detail as much as possible makes
it easier to program.  This commit hides this in one more place than
previously.

It takes advantage of the infrastructure added some releases ago dealing
with the fact that the match of some single characters
case-insensitively can be 2 or even 3 characters.

  "ss" =~ /[ß]/i;

is the most prominent example.

We earlier discovered that /[^ß]/ leads to unexpected behavior, and
using one of these sequences as an endpoint in a range is also unclear
as to what is meant.  This commit leaves existing behavior for those
cases.  That behavior is to use just the first code point in the
sequence for regular [...], and to generate a fatal syntax error for
(?[...]).
regcomp.c: Remove extraneous tests

These two messages used to be warnings, but are now errors, so there is
no need to test which pass they are being output in.
regcomp.c: Don't doubly do 'use encoding'

When reparsing, values have already been converted (if necessary) to
native, so don't do it again.