socket-util: Report fd of -1 on error in inet_open_active().

socket-util: Report fd of -1 on error in inet_open_active().

inet_open_active() is documented to report a fd of -1 when an error occurs.
All three of its callers rely on this, by checking only the fd to determine
whether there was an error.  This means that if the call to
set_nonblocking() or set_dscp() or connect() failed, then the callers would
try to use a fd that had already been closed, wreaking havoc.

This fixes a bug introduced in commit a4efa3fc5d (socket-util: Close socket
on failed dscp modification.)

Bug #13750.
Reported-by: Scott Hendricks <shendricks@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>