nonblocking: reduce dependency

nonblocking: reduce dependency

No need to make nonblocking drag in sockets just for a test;
test them if they are present and skip them otherwise.

* tests/test-nonblocking.c: Only test sockets when in use.
* modules/nonblocking-tests (Depends-on): Drop socket.
(Makefile.am): Link even if sockets are not present.
* modules/pipe2-tests (Makefile.am): Likewise.
* lib/ioctl.c (ioctl) [WIN32]: Fail if sockets are not also in use.

Signed-off-by: Eric Blake <eblake@redhat.com>