EVT backtraces

[bates64]

If a crash happens in a script, we should show the script that crashed rather than the evt_execute_next_command trace

[z64a]

Added EvtCallingLine in evt.c which is NULL except when an EVT_CALL is being processed. When a crash occurs, check this variable and if it isn't NULL it will point to the line in the script which triggered the crash.