regcomp.c: Extract functionality into a static function

This is in preparation for it being used in more than one place in a
future commit.
regcomp.c: Make macro a lookup

The recently introduced macro isMNEMONIC_CNTRL has a look-up and several
tests in it, which occupy time and space.  Since it was only used for
debugging, that did not matter much, but future commits will use it in
more mainline code.  This commit changes it to be a single look-up,
using up one of the spare bits available for that purpose in
PL_charclass.  There are enough available bits that we aren't likely to
run out, really ever.  (We can always add a 2nd word of bits if
necessary.)
regcomp.c: Remove unnecessary cast

The macro does the appropriate cast, and this is slightly more legible.
regcomp.c: Move comment closer to code it applies to
regcomp.c: Don't prematurely skip error checking

The assertion in the comment changed by this commit was true only for
pass1 of the regex compilation; not pass2.  This makes it true in both
passes by moving it, and the code it was about past some error checking.
This error checking was executed in pass1, but not pass2.  It also
changes the warning to only be done in the second pass, part of
[perl #122671].  A future commit will fix the others
regcomp.c: Vertically stack ternary

for legibility
recomp.c: output given warning in only one pass

Otherwise, it will be output multiple times.  This warning was untested
for as well.
PATCH: [perl #122671] Many warnings in regcomp.c can occur twice

This solves the problem by moving the warnings to be output only in
pass2 of compilation.  The problem arises because almost all of pass1
can be repeated under certain circumstances described in the ticket and
the added comments of this patch.
regcomp.c: Refactor one area to use common subroutine

By using the inline function append_utf8_from_native_byte(), the details
of this conversion are hidden from here.  Since that routine advances
the parsing pointer with each byte, this has to be slightly refactored.
regcomp.c: Comment clarifications, nits
regcomp.c: Refactor func so caller handles anomalies

S_grok_bslash_N() is refactored to not know about the strictness level
required by the caller, and to return things instead so that the caller
can decide what action to take.

This is in preparation for some changes in the caller's behavior in
future commits.

This has the effect of changing the parsing position or where a problem
occurs shown in a warning message.
regcomp.c: White-space only

Properly indent code in blocks newly formed by the previous commit