Remove ping domain.

ping in Android no longer requires any additional privileges beyond
the caller.  Drop the ping domain and executable file type entirely.

Also add net_domain() to shell domain so that it can create and
use network sockets.

Change-Id: If51734abe572aecf8f510f1a55782159222e5a67
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>