Global JS exit() function now sets 'exit_code' to 0 by default

Calling exit() causes JS_ExecuteScript() to return false (same return value
as if there's a syntax error) - so if we always set exit_code in exit()
(even when not passed a parameter) and then use that exit code if the property
is defined, then we get the -1 return value from js_execfile() if the script
is terminated prematurely without using exit().

This fixes the issue introduced in the previous commit where scripts that
call exit() - without any parameters, were causing errors to be logged about
scripts (e.g. timed events) returning -1.