MINOR: http-act/tcp-act: Add "set-nice" for tcp content rules

MINOR: http-act/tcp-act: Add "set-nice" for tcp content rules

It is now possible to set the "nice" factor of the current stream from a
"tcp-request content" or "tcp-response content" ruleset. To do so, the
action parsing is moved in stream.c and the action evaluation is handled in
a dedicated function.

This patch may be backported as far as 2.2 if necessary.