Fix compiler warnings in openmrn when using new GCC's. (#772)

There are a few places where a newer GCC used by esp-idf emits a warning for OpenMRN code. This PR fixes those.

The fall through warning unfortunately has false positives, but that's still not a good reason to disable it, we rather adjust the comments here to suit the taste of the compiler. 

===

* Fix compiler warnings in openmrn when using new GCC's.

* Fix comment.

src/executor/Executor.hxx

@@ -165,7 +165,7 @@ public:
 
     /// Helper function for debugging and tracing.
     /// @return currently running executable or nullptr if none active.
-    Executable* volatile current() { return current_; }
+    Executable* current() { return current_; }
 
 protected:
     /** Thread entry point.

src/freertos_drivers/esp32/Esp32Gpio.hxx

@@ -552,8 +552,9 @@ template <class Defs> struct GpioInputPUPD : public GpioInputPin<Defs, true, tru
     struct NAME##Defs                                                          \
     {                                                                          \
         static const gpio_num_t PIN_NUM = (gpio_num_t)NUM;                     \
+                                                                               \
     public:                                                                    \
-        static const gpio_num_t pin()                                          \
+        static gpio_num_t pin()                                                \
         {                                                                      \
             return PIN_NUM;                                                    \
         }                                                                      \

src/openlcb/MemoryConfig.hxx

@@ -670,8 +670,8 @@ private:
                     // Custom spaces cannot do free yet.
                     return respond_reject(Defs::ERROR_INVALID_ARGS);
                 }
-                // Fall through.
             }
+            // Fall through
             case MemoryConfigDefs::COMMAND_ENTER_BOOTLOADER:
             {
                 enter_bootloader();
@@ -733,7 +733,8 @@ private:
                     return exit();
                 }
                 LOG(VERBOSE, "memcfg handler reply: no client registered");
-            } // fall through to unsupported.
+                // fall through to unsupported
+            } // fall through
             default:
                 // Unknown/unsupported command, reject datagram.
                 return respond_reject(Defs::ERROR_UNIMPLEMENTED_SUBCMD);

src/openlcb/SimpleInfoProtocol.hxx

@@ -221,8 +221,8 @@ private:
 #if OPENMRN_HAVE_POSIX_FD
             case SimpleInfoDescriptor::FILE_CHAR_ARRAY:
                 open_and_seek_next_file();
-                // fall through
 #endif
+            // Fall through 
             case SimpleInfoDescriptor::CHAR_ARRAY:
                 byteOffset_ = 0;
                 currentLength_ = d.arg;