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.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index d94bdf2..7843e04 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -12057,6 +12057,7 @@
     - set-dst <expr>
     - set-dst-port <expr>
     - set-log-level <level>
+    - set-nice <nice>
     - set-src <expr>
     - set-src-port <expr>
     - set-var(<var-name>) <expr>
@@ -12112,6 +12113,9 @@
   The "set-log-level" is used to set the log level of the current session. More
   information on how to use it at "http-request set-log-level".
 
+  The "set-nice" is used to set the "nice" factor of the current session. More
+  information on how to use it at "http-request set-nice".
+
   The "set-src" and "set-src-port" are used to set respectively the source IP
   and port. More information on how to use it at "http-request set-src".
 
@@ -12359,6 +12363,11 @@
         session. More information on how to use it at "http-response
         set-log-level".
 
+    - set-nice <nice>
+        The "set-nice" is used to set the "nice" factor of the current
+        session. More information on how to use it at "http-response
+        set-nice".
+
     - set-var(<var-name>) <expr>
         Sets a variable.