Making the transport layer more robust

[Frank Ch. Eigler]

> I tested that Ok, as Android defines PATH correctly. However, I have
> found these comments on a systemtap execvp call so I am no sure of
> possible interferences with other requirements: [...]

On second thought ... the place where we were worried about these
extra syscalls is the path that fork/exec's the CMD part of "stap -c
CMD".  In the case of system_cmd() and cleanup_and_exit(), those don't
matter.

- FChE