procd: show process's exit code

procd: show process's exit code

Adds feature to show exit code of processes launched by procd.
The exit code is shown for finished process when ubus's
service list method is called.

The exit code value is computed according to waitpid(2)
and http://tldp.org/LDP/abs/html/exitcodes.html

Signed-off-by: Ondřej Votava <ondrej.votava@cvut.cz>