Default to socket_device for anything under /dev/socket.

Otherwise sockets that have no specific entry match the /dev(/.*) entry
instead, leaving them in device type rather than socket_device type.
Every socket should get its own entry regardless, but this at least puts
it into a more specific type by default.

Change-Id: I97f7999af7f9f83484d3a51440dda791d3726f1a
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>