init: Don't ignore setsockcreatecon errors

The init language supports setting the creation context of a socket
as the 6th argument to the socket keyword. For example, in the
following service, the context associated with the netd socket
is u:r:netd:s0

service netd /system/bin/netd
    class main
    socket netd stream 0660 root system u:r:netd:s0
    socket dnsproxyd stream 0660 root inet
    socket mdns stream 0660 root system
    socket fwmarkd stream 0660 root inet

The 6 argument form of the socket statement is rarely if ever used,
since the init code supplies a sensible default.

Currently, there's no error checking on the value supplied as
the 6th argument. For example, if you have the following socket
statement:

  socket netd stream 0660 root system graphics

a socket will attempt to get created with an invalid "graphics"
context. When setsockcreatecon fails, it retains the default socket
creation context, which for init is u:r:init:s0. This results in a
socket being created which is in an unexpected context.

Check the return value from the setsockcreatecon() call. If an
invalid context is specified, return early and don't subsequently
attempt to create the socket with the default context.

Bug: 25851205
Change-Id: Ic66cd6f7efe3897fb247b587ddeac5d35e1602b7