metadata: document creds_item_off and auxgrps_item_off fields

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
connection: move the install creds into the slice to its own function

Move the install creds into the receiver's slice to its own function
kdbus_conn_creds_install().

Use from_kuid_munged(), so the uid mapping never fails.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
test: add helpers for user namespace mapping

Will be used by other tests.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
test: move shared definitions to kdbus-util.h to use them in other tests

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
test: add conn_recv() that use poll() and msg_recv()

Will be used by tests.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
test: add the metadata tests inside new namespaces

Currently this only handles user namespace. Use it to inspect the
dumped metadata/credentials.

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>