regcomp.h: Free up bit in ANYOF FLAGS field

regcomp.h: Free up bit in ANYOF FLAGS field

I've long been confronted with trying to do things to create a spare bit
to use.  I thought it easier now, while it's fresh in my mind, to free
up one for future use, rather than re-learn things when it next becomes
necessary.  It would have been a different story if the freed bit had
required a performance penalty.

This commit also updates the comments about how to create even more
spare bits should it become necessary.