pp_subst: don't use REXEC_COPY_STR on 2nd match

pp_subst: don't use REXEC_COPY_STR on 2nd match

pp_subst() sets the REXEC_COPY_STR flag on the first match. On the second
and subsequent matches, it doesn't set it in two out three of the branches
(including pp_susbstcont) where it calls CALLREGEXEC().
The one place where it *does* set it is a (harmless) mistake, since regexec
ignores REXEC_COPY_STR if REXEC_NOT_FIRST is set (which is it is, on all 3
brnanches).

So unset REXEC_COPY_STR in the third branch too, for consistency