socket: Add Support for TCP keep alive variables

The tcp keep alive variables now can be configured via conf
parameter. Follwing variables are now supported by this patch.

tcp_keepalive_intvl: The number of seconds between TCP keep-alive probes

tcp_keepalive_probes: The maximum number of TCP keep-alive probes to
send before giving up and killing the connection if no response is
obtained from the other end.

tcp_keepalive_time: The number of seconds a connection needs to be
idle before TCP begins sending out keep-alive probes.
socket: Add support for TCP defer accept

TCP_DEFER_ACCEPT Allow a listener to be awakened only when data
arrives on the socket. If TCP_DEFER_ACCEPT set on a server-side
listening socket, the TCP/IP stack will not to wait for the final
ACK packet and not to initiate the process until the first packet
of real data has arrived. After sending the SYN/ACK, the server will
then wait for a data packet from a client. Now, only three packets
will be sent over the network, and the connection establishment delay
will be significantly reduced.
socket: add bus property for bus property NoDelay

Missed to add the SD_BUS_PROPERTY for no_delay.