trace-cmd improve main() function

[Federico Vaga]

In two steps I tried to improve the `main()` function. The reason for
those patches is that the current `main()` function is doing much more
than the necessary (implementing commands); this makes the code not
uniform and less clear.

Tests
Since those patches are mainly, copy & paste and re-writing how `trace-cmd`
runs command, I tested these changes by executing the help message for all
the commands. In principle, if we are able to run the commands the patches
are correct.