Added coolant control (M7*, M8, M9). Mist control can be enabled via config.h.

- Added coolant control! Flood control (M8) functions on analog pin 0.
Mist control (M7) is compile-time optional and is on analog pin 1. (Use
only if you have multiple coolants on your system). Based on work by
@openpnp.

- Fixed some variable assignments in spindle control.

coolant_control.c

@@ -0,0 +1,65 @@
+/*
+  coolant_control.c - coolant control methods
+  Part of Grbl
+
+  Copyright (c) 2012 Sungeun K. Jeon
+
+  Grbl is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  Grbl is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with Grbl.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "coolant_control.h"
+#include "settings.h"
+#include "config.h"
+#include "planner.h"
+
+#include <avr/io.h>
+
+static uint8_t current_coolant_mode;
+
+void coolant_init()
+{
+  current_coolant_mode = COOLANT_DISABLE;
+  #if ENABLE_M7
+    COOLANT_MIST_DDR |= (1 << COOLANT_MIST_BIT);
+  #endif
+  COOLANT_FLOOD_DDR |= (1 << COOLANT_FLOOD_BIT);
+  coolant_stop();
+}
+
+void coolant_stop()
+{
+  #if ENABLE_M7
+    COOLANT_MIST_PORT &= ~(1 << COOLANT_MIST_BIT);
+  #endif
+  COOLANT_FLOOD_PORT &= ~(1 << COOLANT_FLOOD_BIT);
+}
+
+
+void coolant_run(uint8_t mode)
+{
+  if (mode != current_coolant_mode)
+  { 
+    plan_synchronize(); // Ensure coolant turns on when specified in program.
+    if (mode == COOLANT_FLOOD_ENABLE) { 
+      COOLANT_FLOOD_PORT |= (1 << COOLANT_FLOOD_BIT);
+    #if ENABLE_M7  
+      } else if (mode == COOLANT_MIST_ENABLE) {
+          COOLANT_MIST_PORT |= (1 << COOLANT_MIST_BIT);
+    #endif
+    } else {
+      coolant_stop();
+    }
+    current_coolant_mode = mode;
+  }
+}