metadata: port ACL support to C

metadata: port ACL support to C

Use some own C code instead of the posix1e python bindings, as those
don't have correct 'bytes' support (at least right now), which means
that we cannot use them with arbitrary file, user and group names.
Our own wrappers just use 'bytes' throughout.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
[rlb@defaultvalue.org: adjust to rely on pkg-config]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>