Allow choice to trap or not program faults

The binder traps faults by default to recover as much as
possible from unexpected errors.

Developers may need to remove that behaviour to get core file.

This introduces 2 ways to set or remove the trapping of faults:

 - the environment variable AFB_TRAP_FAULTS

 - the command line option --trap-faults

Both take a boolean value set using on of the following words:

 1  yes  true   on    -- enable trapping of faults (the default)
 0  no   false  off   -- disable trapping of faults

Bug-AGL: SPEC-1702

Change-Id: I6c6a3b933dbda9922a6079c390a601c533d49e55
Signed-off-by: José Bollo <jose.bollo@iot.bzh>