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.