[FORTRAN PATCH] PR 30404: Rewrite nested FORALL mask expansion

[Paul Thomas]

Roger,

I have given you patch a good look-over; I have tested it but not
regtested - as ususal, it's a bit of very high quality work.  In fact,
it also fixes the second part of Dominique's original bug - if the
outermost forall is unmasked, so to speak, there is no longer a segfault.

For what it's worth, I can add my OK to Steve's.  In fact, I think that
it is a bug of such seriousness that if trunk survives it for a couple
of weeks, 4.2 should be patched too.

Thanks

Paul