Make ready for clixon 5.4.0 development

CONTRIBUTING.md

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+## Documentation
+
+How to document the code::
+
+  /*! This is a small comment on one line
+   *
+   * This is a detailed description
+   * spanning several lines.
+   *
+   * Example usage:
+   * @code
+   *   fn(a, &b);
+   * @endcode
+   *
+   * @param[in] src         This is a description of the first parameter
+   * @param[in,out] dest    This is a description of the second parameter
+   * @retval TRUE           This is a description of the return value
+   * @retval FALSE          This is a description of another return value
+   * @see                   See also this function
+   */
+
+## C style
+
+Clixon uses 4-char indentation, a la emacs "cc-mode".
+
+### Function declarations
+
+Functions in C code are written as follows::
+
+  static int
+  myfn(int           par1,
+       my_structure *par2)
+  {
+      int           retval = -1;
+      my_structure *ms;
+
+      ms = NULL;
+
+Notes:
+
+1. the return type of the function and all qualifers on first line (`static int`)
+2. function name and first parameter on second line, thereafter each parameter on own line
+3. Each parameter indented to match the "longest" (`my_structure`)
+4. Pointer declarations written: `type *p`, not: `type* p`.
+5. All local variables in a function declared at top of function, not inline with C-statements.
+6. Local variables can be initialized with scalars or constants, not eg malloc or functions with return values that need to be  checked for errors
+7. There is a single empty line between local variable declarations and the first function statement.
+
+
+Function signatures are declared in include files or in forward declaration using "one-line" syntax, unless very long::
+
+  static int myfn(int par1, my_structure *par2);
+
+### Errors
+
+Errors are typically declared as follows::
+
+    if (myfn(0) < 0){
+       clicon_err(OE_UNIX, EINVAL, "myfn");
+       goto done;
+    }
+
+All function returns that have return values must be checked
+
+Default return values form a function are:
+
+- `0`  OK
+- `-1` Fatal Error
+
+In some cases, Clixon uses three-value returns as follows:
+
+- `1`  OK
+- `0`  Invalid
+- `-1` Fatal error
+
+### Return values
+
+Clixon uses goto:s only to get a single point of exit functions as follows::
+
+  {
+      int retval = -1;
+  
+      ...
+      retval = 0;
+    done:
+      return retval
+  }
+
+Notes:
+
+1. Use only a single return statement in a function
+2. Do not use of goto:s in other ways
+
+### Comments
+
+Use `/* */`. Use `//` only for temporal comments.
+
+Do not use "======", ">>>>>" or "<<<<<<" in comments since git merge conflict uses that.