CODING_STYLE: allow joined variable declarations and function calls

… but only for a single variable.

The guidelines already allowed declaring variables at the point of
initialization. But not making a function call to do that. Let's allow that
now.  The existing style of declaring and initializing seperate is still
allowed, and whatever makes most sense should be used.

docs/CODING_STYLE.md

@@ -153,25 +153,34 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 ## Using C Constructs
 
 - Allocate local variables where it makes sense: at the top of the block, or at
-  the point where they can be initialized. `r` is typically used for a local
-  state variable, but should almost always be declared at the top of the
-  function.
+  the point where they can be initialized. Avoid huge variable declaration
+  lists at the top of the function.
+
+  As an exception, `r` is typically used for a local state variable, but should
+  almost always be declared as the last variable at the top of the function.
 
   ```c
   {
-          uint64_t a, b;
+          uint64_t a;
           int r;
 
-          a = frobnicate();
-          b = a + 5;
-
-          r = do_something();
+          r = frobnicate(&a);
           if (r < 0)
                   …
+
+          uint64_t b = a + 1, c;
+
+          r = foobarify(a, b, &c);
+          if (r < 0)
+                  …
+
+          const char *pretty = prettify(a, b, c);
+          …
   }
   ```
 
-- Do not mix function invocations with variable definitions in one line.
+- Do not mix multiple variable definitions with function invocations or
+  complicated expressions:
 
   ```c
   {
@@ -225,7 +234,7 @@ SPDX-License-Identifier: LGPL-2.1-or-later
 - To determine the length of a constant string `"foo"`, don't bother with
   `sizeof("foo")-1`, please use `strlen()` instead (both gcc and clang optimize
   the call away for fixed strings). The only exception is when declaring an
-  array. In that case use STRLEN, which evaluates to a static constant and
+  array. In that case use `STRLEN()`, which evaluates to a static constant and
   doesn't force the compiler to create a VLA.
 
 - Please use C's downgrade-to-bool feature only for expressions that are