Allow nghook to execute a program with the data socket connected to

Allow nghook to execute a program with the data socket connected to
stdin and stdout instead of relaying the data. Now it is possible
to say:

	nghook -e path: hook /usr/local/bin/foo arg1 arg2

and foo will have the hook to path: at file descriptors 0 and 1.

Add an option to specify control messages to be send to the node before
either executing the program or entering the data relay loop.