Changes

stream_srv: srv_link: Set OSMO_SOCK_F_NONBLOCK during socket creation

This should be needed to potentially avoid blocking during accept()
call.

In practice right now is not needed because osmo_fd/osmo_iofd takes care
of setting to non-blocking during registering:
* osmo_fd: osmo_fd_register() sets the fd to non-bloking.
* osmo_iofd:
** osmo_io_poll: it uses osmo_fd_register() internally, see above
** osmo_io_uring: only set to non-blocking as a side-effect of calling
   osmo_fd_register() if IOFD_FLAG_NOTIFY_CONNECTED was passed.

Since for the srv_link so far always uses an osmo_fd regardless of
whether OSMO_STREAM_MODE_* is used, in practice we are safed by
osmo_fd_register().

In any case, better pass the flag at socket creation time so it's
already applied then, instead of relying on osmo_fd_register() as a
side-effect, specially if in the future we switch the listen socket to
be operated over osmo_io_uring.

Change-Id: I98a6ef55a0a7289b9eeb7e8062e9afa4d87f51eb