tc: f_flower: add support for matching on tunnel metadata

tc: f_flower: add support for matching on tunnel metadata

extend TC flower for matching on tunnel metadata.

Changes since v2:
 - split uAPI changes and TC code in separate patches, as per David's request [2]

Changes since v1:
 - fix incostintent naming in explain() and in tc-flower.8 (Asbjørn)

Changes since RFC:
 - update uAPI bits to Asbjørn's most recent code [1]
 - add 'tun' prefix to all flag names (Asbjørn)
 - allow parsing 'enc_flags' multiple times, without clearing the match
   mask every time, like happens for 'ip_flags' (Asbjørn)
 - don't use "matches()" for parsing argv[]  (Stephen)
 - (hopefully) improve usage() printout (Asbjørn)
 - update man page

[1] https://lore.kernel.org/netdev/20240709163825.1210046-1-ast@fiberby.net/
[2] https://lore.kernel.org/netdev/cc73004c-9aa8-9cd3-b46e-443c0727c34d@kernel.org/

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>