re_intuit_start(): introduce rx_origin var
re_intuit_start() is a bit of mess. It uses two general function-scope
vars, s and t, to point at string offsets while processing. These vars
mean different things at different times. Introduce a new var, rx_origin,
which indicates the current minimum position that the regex could begin
matching at. It starts off at strpos, and gradually moves up as various
constraints are rejected. It will be the value eventually returned.
For the moment, s and/or t will continue serving that function at various
points in the code; this commit just makes rx_origin valid at the entry to
the 'restart:' block.