bup: add own gethostname() wrapper

This is necessary because python3 insists that hostnames should
be utf-8, which is a rather questionable assumption.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
[rlb@defaultvalue.org: don't define HOST_NAME_MAX if it's not already]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>