librgw: introduce RGW_LOOKUP_FLAG_CREATE

librgw: introduce RGW_LOOKUP_FLAG_CREATE

The current API relies on rgw_lookup and rgw_lookup_filehandle to
return file handle instances, and like POSIX, needs to deal in some
way with the NFS/POSIX open-for-create concept.

For now, allow rgw_lookup to return the canonical instance for an
object being created, i.e., if the option RGW_LOOKUP_FLAG_CREATE
is passed.  (I added an RGW_OPEN_FLAG_CREATE also, but it doesn't
do anything.)

Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>