socket-util: Allow binding without a port number in inet_open_passive().

socket-util: Allow binding without a port number in inet_open_passive().

The test-vconn program binds a socket to a nonspecific port number.  To
add SSL support to this program, it needs to be able to use SSL, and the
stream library is the easiest way to do that.  But the stream library
can't bind to a nonspecific port.  This commit adds that feature, by adding
it to the function that the stream SSL library uses as a building block.