regcomp.c: Move bit to different data structure

regcomp.c: Move bit to different data structure

Commit 899d20b99829f8ecdc14e1351b533bc62a354dea was used to free up a
bit in a flags field that had run out of bits at the time.  Further work
has made that unnecessary, and this commit moves it back to the flags
field, which even after this commit has a spare bit (which is intended
to be used in a future commit).

Doing so makes this bit "just one of the guys", so can be operated on
en-masse with the others.  This allows a little code to be removed, and
the knowledge of this flag mostly confined to lower level subroutines.