Add a coccinelle script to look for {inc,dec}rements in log_debug

We want to forbid this pattern since, unlike the other log_*()
macros, log_debug() conditionally evaluates its arguments only if
debug-level logging is enabled.  Thus, a call to
    log_debug("%d", x++);
will only increment x if debugging logs are enabled, which is
probably not what the programmer intended.

One bug caused by this pattern was #30628.

This script detects log_debug( ) calls with any of E++, E--, ++E,
or --E in their arguments, where E is an arbitrary expression.

Closes ticket 30743.

changes/ticket30743

@@ -0,0 +1,7 @@
+  o Minor features (maintenance scripts):
+    - Add a coccinelle script to detect bugs caused by incrementing or
+      decrementing a variable inside a call to log_debug().  Since
+      log_debug() is a macro whose arguments are conditionally evaluated, it
+      is usually an error to do this. One such bug was 30628, in which SENDME
+      cells were miscounted by a decrement operator inside a log_debug()
+      call.  Closes ticket 30743.