Try to unmount writable filesystems when rebooting

Ext4 filesystems like to be unmounted before rebooting.  The Android system
doesn't have a traditional Linux init setup, and shutting down the system
was not much more than calling sync(2) and reboot(2).  This adds a new
function to libcutils called android_reboot().  By default, it calls sync()
and then remounts all writable filesystems as read-only and marks them clean.
There is a flag parameter in which the caller can ask for sync() not to be
called, or to not remount the filesystems as read-only.  Then it will call
reboot(2) as directed by the other parameters.  This change also updates
adb, init and toolbox to call the new android_reboot() function.
Fixes bugs 3350709 and 3495575.

Change-Id: I16d71ffce3134310d7a260f61ec6f4dd204124a7
diff --git a/libcutils/Android.mk b/libcutils/Android.mk
index 3dc3d69..b6c5382 100644
--- a/libcutils/Android.mk
+++ b/libcutils/Android.mk
@@ -109,7 +109,7 @@
 # ========================================================
 include $(CLEAR_VARS)
 LOCAL_MODULE := libcutils
-LOCAL_SRC_FILES := $(commonSources) ashmem-dev.c mq.c
+LOCAL_SRC_FILES := $(commonSources) ashmem-dev.c mq.c android_reboot.c
 
 ifeq ($(TARGET_ARCH),arm)
 LOCAL_SRC_FILES += arch-arm/memset32.S