info: Prefer NBD_OPT_INFO when possible

Now that libnbd supports extended headers, it is worth observing that
the qemu implementation for NBD_CMD_BLOCK_STATUS that supports an
extended header for filtering the server's response, as advertised by
NBD_FLAG_BLOCK_STAT_PAYLOAD, is conditionally advertised.  When using
NBD_OPT_GO, the flag is only advertised if the client requested more
than one meta context (as otherwise, there is no use filtering).  But
for NBD_OPT_INFO, the flag is advertised if extended headers are
enabled, regardless of whether meta contexts are negotiated yet.

In order for upcoming libnbd patches to add support for probing and
using this bit reliably, we therefore need 'nbdinfo --can ...'  to
favor the information learned by NBD_OPT_INFO instead of NBD_OPT_GO,
because that is lighter weight than figuring out whether an export
supports at least two meta contexts that can then be negotiated.

Do this by changing nbdinfo to prefer opt mode always, then touching
up the few places (--map, --content) that need to ensure NBD_OPT_GO.
This is made easier by the previous patches that make nbd_shutdown()
work sanely regardless of whether we are still in opt mode.

In turn, this patch flushed out a double-free bug in 'nbdkit file
dir=...' when querying opt_info on the default name, so a couple of
unit tests need to avoid false negatives on platforms where nbdkit
commit 39d62de9 is not yet backported.

Signed-off-by: Eric Blake <eblake@redhat.com>