MEDIUM: tcp-act: <expr> support for set-fc-{mark,tos} actions

MEDIUM: tcp-act: <expr> support for set-fc-{mark,tos} actions

In this patch we add the possibility to use sample expression as argument
for set-fc-{mark,tos} actions. To make it backward compatible with
previous behavior, during parsing we first try to parse the value as
as integer (decimal or hex notation), and then fallback to expr parsing
in case of failure.

The documentation was updated accordingly.