eliminate PL_regsize

eliminate PL_regsize

This var (or rather PL_reg_state.re_state_regsize, which it is #deffed to)
just holds the index of the maximum opening paren index seen so far in
S_regmatch(). So make it a local var of S_regmatch() and pass it as a
param to the couple of static functions called from there that need it.

(Also give the local var the more meaningful name 'maxopenparen'.)