Move the external program "pause on exit" implementation back to the C++ code.
Originally, the "pause on exit" was implemented in the C++ version of sbbs_t::xtrn_sec() and not sbbs_t::exec_xtrn(). This meant that the pause option was only implemented when the prgoram was invoked from the menu and not when invoked directly (e.g. via Baja or JS exec_xtrn function). When sbbs_t::xtrn_sec() was ported to JS, that functionality was also moved to JS. But it really should have been implemented in exec_xtrn() to begin with so that regardless of how the program is invoked, that SCFG setting takes effect. Thanks to Michael Long for pointing this out.