Add support for TCP keepalive (-K option)
Add the -K option to help and man page
Merge branch 'keepalive'

Conflicts:
	src/netcore.c
set return value to EXIT_SUCCESS on successful completion of NETCAT_LISTEN mode
Merge branch 'sourceforge-bug-47'
Only set SO_LINGER on TCP sockets

SO_LINGER is meaningless for UDP and on some platforms, an attempt to
set that option on a UDP socket triggers an error (see Sourceforge
issues #50, #55 and #56).
Merge branch 'master' into sourceforge-bug-56

Conflicts:
	src/network.c