Eliminate the duplicative regops BOL and EOL

See also perl5porters thread titled: "Perl MBOLism in regex engine"

In the perl 5.000 release (a0d0e21ea6ea90a22318550944fe6cb09ae10cda)
the BOL regop was split into two behaviours MBOL and SBOL, with SBOL
and BOL behaving identically. Similarly the EOL regop was split into
two behaviors SEOL and MEOL, with EOL and SEOL behaving identically.

This then resulted in various duplicative code related to flags and
case statements in various parts of the regex engine.

It appears that perhaps BOL and EOL were kept because they are the
type ("regkind") for SBOL/MBOL and SEOL/MEOL/EOS. Reworking regcomp.pl
to handle aliases for the type data so that SBOL/MBOL are of type
BOL, even though BOL == SBOL seems to cover that case without adding
to the confusion.

This means two regops, a regstate, and an internal regex flag can
be removed (and used for other things), and various logic relating
to them can be removed.

For the uninitiated, SBOL is /^/ and /\A/ (with or without /m) and
MBOL is /^/m. (I consider it a fail we have no way to say MBOL without
the /m modifier). Similarly SEOL is /$/ and MEOL is /$/m (there is
also a /\z/ which is EOS "end of string" with or without the /m).
change NODE_ALIGN_FILL to set flags to 0

In 075abff3 Andy Lester set the flags field of regops
to default to 0xde. I find this really weird, and possibly dangerous,
as it seems to me reasonable to assume a new regop would have this
field set to 0, so that later on code can set it to something else
if necessary. (Which is what I wanted to do.)

Since nothing breaks if I set it to 0x0 and I find that to be a much
more natural default than 0xde (the prefix of 0xdeadbeef), I am
changing this to set it to 0.
Perl RT #122761 - split /\A/ should not behave like split /^/m

Long ago a weird special case was hacked into split so that it
treated C<split /^/> as if it was C<split /^/m>. At the time this
was done by letting the split PP code inspect the pattern, and IFF
it matched "^\0" the special behavior was enabled (which also
bypasses using the regex engine for matching.)

Later on when we added pluggable regex engines and when we
encountered various counter-intuitive behaviors related to split
we changed who this worked so that the regex engine would
set flags appropriate for split to use. This meant that regex
plugins using totally different regex syntax could still enable
the optimisation. At the same time I modified how we detected
this pattern type by looking at the *compiled* regops, and not
the raw pattern. This had the side effect of making things like
C< split /(?:)^/ > also enable the optimisation.

Unfortunately this did not play nicely with the fact that /^/
produces an SBOL node, as does /\A/, but we definitely don't want
C<split /\A/> to behave like C<split /^/m>. In fact C<split /\A/>
should behave like a noop (which means there is room for a future
optimisation here if someone cares to implement it.)

In the discussion attached to the ticket I propose what I consider
to be a better fix, default split patterns to be compiled by default
with the /m modifier enabled. This patch does NOT do this. It is
instead the "simple" patch. This means that C<split /^/> behaves like
C<split /^/m> but C<split /^x/> does NOT behave like C<split /^x/m>
which I consider to be a bug which I will fix in a future patch.