Revert "Fix mosh-server segfault on startup when run without arguments"

This reverts commit 578db45fbf65d4e90bba371fe8d55f0455193a27.

The NULL inputs to getaddrinfo are valid in certain cases, and at
least in the case of node, important.  The segfault happens not with
getaddrinfo, but with the error message generation below it.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
AddrInfo: Fix error message generation when node == NULL

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Connection::try_bind: Pass port argument 0 as a string

Signed-off-by: Anders Kaseorg <andersk@mit.edu>