regcomp.c: Use old paradigm in dealing with flags recursively

regcomp.c: Use old paradigm in dealing with flags recursively

In a recursive call to reg(), instead of passing our flags pointer, pass
a new one and upon return or in that result with the existing one.  I
can see why this should be done, as you don't want to lose what you
already have, as reg() will start by resetting it to 0.  I don't know
why one ands it with the known flags, but I'm presuming there is a
reason, and so am copying the paradigm.  I searched the commit messages
and didn't find anything.  No tests failed, and I didn't figure out a
test that would fail.