Over time, dlmfs has added some features that were not part of the
initial ABI.  Unfortunately, some of these features are not detectable
via standard usage.  For example, Linux's default poll always returns
POLLIN, so there is no way for a caller of poll(2) to know when dlmfs
added poll support.  Instead, we provide this list of new capabilities.

Capabilities is a read-only attribute.  We do it as a module parameter
so we can discover it whether dlmfs is built in, loaded, or even not
loaded (via modinfo).

The ABI features are local to this machine's dlmfs mount.  This is
distinct from the locking protocol, which is concerned with inter-node
interaction.
Signed-off-by: NJoel Becker <joel.becker@oracle.com>