Reliably initialise uxnet's socket fd fields to -1.

Reliably initialise uxnet's socket fd fields to -1.

This prevents embarrassing mess-ups involving getting back a Socket
which has mostly been memset to 0 but contains an error message,
sk_close()ing it to free the memory, and finding that standard input
has been closed as a side effect.

[originally from svn r10073]