The existing shutdown() code only worked with AF_INET sockets, and returned
ENOTSOCK for AT_LOCAL sockets, because we implemented the latter sockets in
completely different code (in af_local.cc).

So in uipc_syscalls_wrap.c, the same place we call a the special af-local
socketpair(), we also need to call the special af-local shutdown().

The way we do it is a bit ugly, but effective: shutdown() first calls
shutdown_af_local(), and if that returns ENOTSOCK (so it's not an af_local
socket), we continue trying the regular socket shutdown code.

A better way would have been to add shutdown() to the fileops table -
either the generic one (why not?), or invent a new mechanism whereby
certain file types (in this case, "sockets" of all types) can have additional
ops tables including in this case a shutdown() operation. Linux has
something of this sort for implementing shutdown().