Remove Deuce's jsexec object, define js.startup_dir instead, which is now

(potentially) non-blank for external JS modules configured in SCFG->External
Programs, as well as those executed from jsexec. If the startup_dir is
specified in SCFG and no path is given for the .js file, it will look in the
startup_dir first. js.startup_dir will be an empty string for all other cases
(currently).