dbus-send: pretty-print GVariant-style bytestrings

dbus-send: pretty-print GVariant-style bytestrings

dbus-send could already pretty-print bytestrings that do not have
\0 termination, but those are awkward to work with (they need copying),
so they are now discouraged. Teach it to print bytestrings that
do have \0 termination as well.

In the process, rewrite this part of the message parser
to use dbus_message_iter_get_fixed_array(), which is the Right way
to get arrays of numbers out of a message.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=89109
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>