manager: use target process context to set socket context

manager: use target process context to set socket context

Use target process context to set socket context when using SELinuxContextFromNet
not systemd's context. Currently when using the SELinuxContextFromNet option for
a socket activated services, systemd calls getcon_raw which returns init_t and
uses the resulting context to compute the context to be passed to the
setsockcreatecon call. A socket of type init_t is created and listened on and
this means that SELinux policy cannot be written to control which processes
(SELinux types) can connect to the socket since the ref policy allows all
'types' to connect to sockets of the type init_t. When security accessors see
that any process can connect to a socket this raises serious concerns. I have
spoken with SELinux contributors in person and on the mailing list and the
consensus is that the best solution is to use the target executables context
when computing the sockets context in all cases.

[zjs review/comment:

This removes the branch that was added in 16115b0a7b7cdf08fb38084d857d572d8a9088dc.
16115b0a7b7cdf08fb38084d857d572d8a9088dc did two things: it had the branch here
in 'socket_determine_selinux_label()' and a code in 'exec_child()' to call
'label_get_child_mls_label(socket_fd, command->path, &label)'.

Before this patch, the flow was:
'''
mac_selinux_get_child_mls_label:
  peercon = getpeercon_raw(socket_fd);
  if (!exec_label)
     exec_label = getfilecon_raw(exe);

socket_open_fds:
  if (params->selinux_context_net)                 #
     label = mac_selinux_get_our_label();          #  this part is removed
  else                                             #
     label = mac_selinux_get_create_label_from_exe(path);
  socket_address_listen_in_cgroup(s, &p->address, label);

exec_child():
   exec_context = mac_selinux_get_child_mls_label(fd, executable, context->selinux_context);
   setexeccon(exec_context);
'''
]