Set sensible default for port

[waveform80]

Commit 915444bc0b8a931d32dfb755542f4bd1d37f1449 introduced a regression whereby an entry in nbdtab with no port specification was read as wanting port "0" rather than the default "10809".

I attempted to run the current master branch within a test VM but could not get it to connect to the test server using configuration stored in nbdtab. However, using command line options worked happily; strace of the process eventually confirmed it was attempting to connect to port 0 rather than the expected port 10809 which pointed to a configuration parsing error.