Adds options --no-httpd and --exec

Adds options --no-httpd and --exec

The option option --exec use the remaining arguments
of the command line (at the right of --exec) as a
command to launch with its arguments. Arguments can
use @p (for port), @t (for token) or @@ for @.

When the option --exec is given, the command is launched
as soon as afb-daemon is ready. When the command die,
afb-daemon exits and before exiting, afb-daemon kills
the launched command and its childs.

The option --no-http forbids the activation of
the HTTP server.

Change-Id: Icdd91d84c818796b5ac6ea5d33ff3549a2fe3c25
Signed-off-by: José Bollo <jose.bollo@iot.bzh>