eliminate PL_reginput

eliminate PL_reginput

PL_reginput (which is actually #defined to PL_reg_state.re_state_reginput)
is, to all intents and purposes, state that is only used within
S_regmatch().

The only other places it is referenced are in S_regtry() and S_regrepeat(),
where it is used to pass the current match position back and forth between
the subs.

Do this passing instead via function args, and bingo! PL_reginput is now
just a local var of S_regmatch().