Tweak the tracing flags slightly, and clean up error handling and diagnostics
Tracing flags are now:
-l[flags] Log events in binary format to the file <program>.eventlog
-v[flags] Log events to stderr
where [flags] can contain:
s scheduler events
t add time stamps (only useful with -v)
and there are more helpful error messages when using flags that are
only available in particular variants of the RTS:
the flag -v requires the program to be built with -debug
the flag -Ds requires the program to be built with -debug
the flag -p requires the program to be built with -prof
the flag -N requires the program to be built with -threaded
the flag -v requires the program to be built with -debug
Also, I fixed the crash reported in #3874, with +RTS -ls -v.