Rework diagnostics in the regex engine

This introduces three new subs:

Perl_re_printf() which is a wrapper for

    PerlIO_printf( Perl_debug_log, ... ),

which cuts down on clutter in the code. Arguably this could be moved
to util.c and renamed something like PerlIO_debugf() and then we could
declutter all the statements that write to the Perl_debug_log
filehandle. But that is a bit too ambituous for me right now, so
I leave this as a regex engine only sub for now.

Perl_re_indentf() which is a wrapper for PerlIO_re_printf(),
which adds an indent argument and automatically indents the
line appropriately, and is used in regcomp.c for trace diagnostics
during compilation.

Perl_re_indentfo() which is similar to Perl_re_indentf() but
is used in regexec.c which adds a specific prefix to each indented
line to account for the fact that during execution we normally have
string position information on the left.

The end result of this patch is that a lot of clutter in the debugging
statements in the regex engine is reduced, exposing what is actually
going on. It should also now be easier to add new diagnostics which
"do the right thing".

Over time the debugging trace output in regexec has become
very cluttered and confusing. This patch cleans much of it up,
if something happens at a given recursion depth it is output
at the right depth, etc, and formats have been changed to not have
leading spaces so you can actually see the indentation properly.

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