rgw: s3: implement GetBucketPolicyStatus API

rgw: s3: implement GetBucketPolicyStatus API

This API returns whether the Bucket Policies/ACLs are public. There are a couple
of caveats:
- AWS currently returns PolicyNotFound error in case a bucket policy doesn't
exist, though a non existant bucket policy would mean the default ACLs apply
where the bucket is private, so error return here seems like an error
- the API spec mentions TRUE and FALSE as the response IsPublic element value,
however in practice both boto/aws clients and AWS S3 return/expect a lowercase
response.

Signed-off-by: Abhishek Lekshmanan <abhishek@suse.com>
Conflicts:
	src/rgw/rgw_rest_s3.h
merge conflict after zipper rework, dropped a spurious newline in rgw_rest_s3.h
after get_obj_op decl.
src/rgw/rgw_common.h
src/rgw/rgw_rest_s3.cc
src/rgw/rgw_rest_s3.h:
merge conflict after bucket replication merge, trivial conflicts