server: correct comment typo on port being after ':'

We use commas in fio, since that works equally well on IPv4
and IPv6.

Signed-off-by: Jens Axboe <axboe@fb.com>
server: dump where we attempted to bind in case of failure

Helps locate what the issue is, instead of having to use
strace and similar tools.

Signed-off-by: Jens Axboe <axboe@fb.com>