Add support for socket security context specification.

Add an optional argument to the socket option for specifying
a SELinux security context for the socket.  Normally the socket
security context is automatically computed from the service security
context or set using the seclabel option, but this facility allows
dealing with two scenarios that cannot be addressed using the existing
mechanisms:
1) Use of logwrapper to wrap a service.
In this case, init cannot determine the service security context
as it does not directly execute it and we do not want logwrapper
to run in the same domain as the service.

2) Situations where a service has multiple sockets and we want to
label them distinctly.

Signed-off-by: Humberto Borba <humberos@gmail.com>
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Change-Id: I7ae9088c326a2140e56a8044bfb21a91505aea11