add option --T_dont_fork to tracee (lte-softmodem, oaisim)

The T has 2 processes: the tracee and the local tracer
if the tracee dies (segfault or whatever) the tracee
may still be running.

So there is a third process that monitors those two others
and kill everything when one dies.

The problem is that this third process has to be the parent
of the two others. That makes debugging the tracee with gdb
impossible. This new option disables the third monitor process.