Describe the flower control flags, and use them
for key-flags and key-flags-mask.

The flag names have been taken from iproute2.
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20240713021911.1631517-3-ast@fiberby.netSigned-off-by: Jakub Kicinski <kuba@kernel.org>