Make shutdown() return ENOTCONN as required by POSIX, part deux.

Summary:
Back in 2005, maxim@ attempted to fix shutdown() to return ENOTCONN in case the socket was not connected (r150152). This had to be rolled back (r150155), as it broke some of the existing programs that depend on this behavior. I reapplied this change on my system and indeed, syslogd failed to start up. I fixed this back in February (279016) and MFC'ed it to the supported stable branches. Apart from that, things seem to work out all right.

Since at least Linux and Mac OS X do the right thing, I'd like to go ahead and give this another try. To keep old copies of syslogd working, only start returning ENOTCONN for recent binaries.

I took a look at the XNU sources and they seem to test against both SS_ISCONNECTED, SS_ISCONNECTING and SS_ISDISCONNECTING, instead of just SS_ISCONNECTED. That seams reasonable, so let's do the same.

Test Plan:
This issue was uncovered while writing tests for shutdown() in CloudABI:

https://github.com/NuxiNL/cloudlibc/blob/master/src/libc/sys/socket/shutdown_test.c#L26

Reviewers: glebius, rwatson, manpages, gnn, network

Reviewed By: gnn, network

Subscribers: bms, mjg, imp

Differential Revision: https://reviews.freebsd.org/D3039