js.exec() will now execute scripts from the configured mods and exec dirs

... if not passed a full path and the script file doesn't exist in the
startup_dir (if specified) or the same directory the calling script was
executed from (js.exec_dir).

This should fix the reported issue when trying to use the "spy" command from
a cmdshell script run from the mods dir:

!JavaScript  /sbbs/exec/str_cmds.js line 388: Error: Script file (mqtt_spy.js) does not exist