shared/acl-util: convert rd,wr,ex to a bitmask

I find this version much more readable.

Add replacement defines so that when acl/libacl.h is not available, the
ACL_{READ,WRITE,EXECUTE} constants are also defined. Those constants were
declared in the kernel headers already in 1da177e4c3f41524e886b7f1b8a0c1f,
so they should be the same pretty much everywhere.