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.