httpd: make the session timeout configurable

The session timeout was a compile-time constant (200 s), so changing it
meant rebuilding the firmware. Make it a variable that keeps the same
default and add a "session <seconds>" command, so it can be set from
the console or from the startup configuration like the other settings.

Measured on KP_9000_9XHML_X_V3_1: ROM +180 bytes (BANK1 +17, BANK2
+161) and xdata +2 bytes; the internal RAM map is unchanged.

README.md

@@ -344,6 +344,7 @@ netmask zzz.zzz.zzz.zzz = Network mask of the switch
 port x name xxx         = Name xxx the port number x
 port z 1g               = Set 1g speed for port z
 igmp on/off             = Turn IGMP on or off
+session xxxx            = Web session timeout in seconds (default 200)
 ```
 [To be continue]