Support an optional ini filename argument, write to DOSXTRN.ERR

If a 4th optional argument is provided, use that as the ini filename for virtual UART/FOSSIL driver settings instead of "sbbsexec.ini". Using this feature requires an updated sbbsexec.dll that supports an ini-filename argument (instead of a directory) to the VDD_LOAD_INI_FILE command.

If DOSXTRN fails to execute the passed command-line, _spawnvp() returns -1, log the errno value and description to DOSXTRN.ERR (in the same directory as DOSXTRN.ENV). This will help diagnose issues executing DOS command-lines (e.g. file not found).

Upon failure to open/create DOSXTRN.RET, don't prematurely terminate as that would skip a lot of important cleanup.