close update package before installing; allow remount

Close the update package before invoking the binary, to allow the
installer to unmount /cache if it wants to.  Add a function to allow
remounting of a mount as read-only.

Change-Id: Idfcc96c3da66083295177f729263560be58034e4
diff --git a/updater/updater.c b/updater/updater.c
index 6537a94..aa626d2 100644
--- a/updater/updater.c
+++ b/updater/updater.c
@@ -136,7 +136,9 @@
         free(result);
     }
 
-    mzCloseZipArchive(&za);
+    if (updater_info.package_zip) {
+        mzCloseZipArchive(updater_info.package_zip);
+    }
     free(script);
 
     return 0;