regcomp.c: Change #ifdef

This function is currently #ifdef'd out.  Change it so that enabling it
in embed.fnc automatically enables it here as well, making a 2 step
process into just a single step.
regcomp.c: Fix potential scalar leak

The lines in this code were reversed.  We need to check something before
overwriting it, rather than the other way around.  The result would be
that under certain circumstances a SV would not get freed.  Those
circumstances are very limited: the first of the three parameters to
this function is not empty, but the 2nd is, and the output (3rd
parameter) is to overwrite the 2nd.  I found this bug by code reading; I
have searched the code space and there are no current calls to it that
have this parameter configuration, therefore there is no test that can
be added to trigger it.
Regen t/porting/known_pod_issues.dat.

This silences a warning that shows up under pedantic mode
regcomp.c: Extract duplicated code into single fcn

This code that appears twice is nearly duplicate.
regcomp.c: Change debug output to use \t, etc instead of hex

It is easier to read the standard abbreviations \t, \n, etc than the hex
equivalents, \x09, ...
regcomp.c: White-space only

This indents properly to correspond to a newly formed block
regcomp.c: Debug output clearer ranges

It's not immediately obvious what the character class [!-~] matches.
Better is its equivalent: [\x21-\x7e].  This commit changes the debug
output to be the latter for character class matches, while retaining the
current behavior where it is clear what the range matches, in, e.g.,
[J-R].  Ranges like [A-z] include more than just alphabetics, so they
are now output as [\x41-\x7a].  (Debug output is done, for example, when
the command line option -Dr is specified.)
regcomp.c: Change Debug output of char classes

This commit causes the debug output that was formerly "\x4ff", for
example to be \x{4f}f.  It always puts braces around the hex to separate
it from other characters.
regexec.c: Add, clarify comments
regcomp.c: Remove redundant code

This code is redundant, attempting to output what isn't returned in 'lv'
by reglcass_swash(), but that function makes sure that 'lv' contains
everything it should, so we ended up processing (and outputting) the
same data twice.
regcomp.c: White-space only

Outdent code which the previous commit removed from a block.
regcomp.c: Remove extraneous debug info

Prior to this commit the prhase {unicode} was emitted to mark what a
bracketed character class matched that wasn't in that classes bitmap.
This was oftern accompanied by another phrase that gave further
details.  Since everything is {unicode}, the first phrase isn't very
helpful.  Now it is changed to {utf8} for those things that won't match
unless the target string is in utf8 (this includes some upper latin1
code points under /d matches), or {outside bitmap} for where utf8 isn't
necessasily required (this happens for user-defined Unicode properties
that aren't known at compile time).