[IB] uverbs: ABI-breaking fixes for userspace verbs

Introduce new userspace verbs ABI version 3.  This eliminates some
unneeded commands, and adds support for user-created completion
channels.  This cleans up problems with file leaks on error paths, and
also makes sure that file descriptors are always installed into the
correct process.

Signed-off-by: Roland Dreier <rolandd@cisco.com>