adding preference option for dynamic input blocking

/inpblock is now having subcommands 'timeout' and 'dynamic'
with:

/inpblock timeout <milliseconds>

and

/inpblock dynamic <on|off>

Defaults are:

/inpblock timeout 500
/inpblock dynamic on

To get the old behavior specify:

/inpblock timeout 20
/inpblock dynamic off

The dynamic mode will block incrementally after something
should be written to the window or after a key was pressed. So pressing
a key would set the timeout to 0ms and after 10 timeouts to the next
bigger one.

Example (with dynamic mode on):

"/inpblock timeout 50"

timeout series:

10x 0ms
10x 10ms (0ms + 10 times since last keypress)
10x 30ms (10ms + 20 times since last keypress)
*x50ms until next key was pressed or

src/command/command.c

@@ -623,14 +623,20 @@ static struct cmd_t command_defs[] =
           NULL } } },
 
     { "/inpblock",
-        cmd_inpblock, parse_args, 1, 1, &cons_inpblock_setting,
-        { "/inpblock millis", "Input blocking delay.",
-        { "/inpblock millis",
+        cmd_inpblock, parse_args, 2, 2, &cons_inpblock_setting,
+        { "/inpblock [timeout|dynamic] [millis|on|off]", "Input blocking delay (dynamic or static).",
+        { "/inpblock [timeout|dynamic] [millis|on|off]",
           "----------------",
-          "Time to wait in milliseconds before reading input from the terminal buffer, defaults to 20.",
-          "Valid values are 1-1000.",
-          "A higher value will result in less CPU usage, but a noticable delay for response to input.",
-          "A lower value will result in higher CPU usage, but faster response to input.",
+          "Setting for how long to wait for input before checking for new messages or checking for state",
+          "changes like 'idle'.",
+          "timeout : Time to wait in milliseconds before reading input from the terminal buffer, defaults to 500.",
+          "        : Valid values are 1-1000.",
+          "dynamic : Either use the timeout statically or use a dynamic variant which is responsive after an input",
+          "        : and blocks longer (up to the specified 'timeout').",
+          "        : on|off",
+          "A higher value will result in less CPU usage (irrespective if dynamic is on or off), but a noticable",
+          "delay for response to input if dynamic is off.",
+          "A lower value will result in higher CPU usage, but faster response to input if dynamic is off.",
           NULL } } },
 
     { "/notify",