regcomp.c: Use strnEQ instead of series of tests

At some point it is more efficient and certainly clearer to call a
library function to do a strcmp than to test each individual byte.
Based on irc discussion with the original coder of this, I changed this
sequence of 6 comparisions to a strnEQ.  At the same time, this adds an
buffer overlflow check.  I was unable to cause the previous code to
overflow, but I believe it was possible.  And this changes the magic
number 6 in the code to a mnemonic giving its meaning.
regcomp.c: Move macro definition earlier in file

This is to prepare it to be used in earlier places than it is now.
Tighten uses of regex synthetic start class

A synthetic start class (SSC) is generated by the regular expression
pattern compiler to give a consolidation of all the possible things that
can match at the beginning of where a pattern can possibly match.
For example
    qr/a?bfoo/;
requires the match to begin with either an 'a' or a 'b'.  There are no
other possibilities.  We can set things up to quickly scan for either of
these in the target string, and only when one of these is found do we
need to look for 'foo'.

There is an overhead associated with using SSCs.  If the number of
possibilities that the SSC excludes is relatively small, it can be
counter-productive to use them.

This patch creates a crude sieve to decide whether to use an SSC or not.
If the SSC doesn't exclude at least half the "likely" possiblities, it
is discarded.  This patch is a starting point, and can be refined if
necessary as we gain experience.

See thread beginning with
http://nntp.perl.org/group/perl.perl5.porters/212644

In many patterns, no SSC is generated; and with the advent of tries,
SSC's have become less important, so whatever we do is not terribly
critical.