Mount the new system as 'postinstall_file' in postinstall.

When mounting the new filesystem on /postinstall, we need to override
the file attributes from the new system (unknown to the current selinux
policies) with a consistent label that will be used only in the context
of postinstall. This patch passes an extra option to mount(2) in
Brillo and Android to achieve this.

Bug: 27177071
TEST=Deployed a postinstall script with `ls -laZ`, contents show "postinstall_file".

Change-Id: Ia43b45c92e4c4cd340a884818ac00f24a418f9e7
diff --git a/common/utils.h b/common/utils.h
index 8da0726..df06ef1 100644
--- a/common/utils.h
+++ b/common/utils.h
@@ -177,7 +177,8 @@
 bool MountFilesystem(const std::string& device,
                      const std::string& mountpoint,
                      unsigned long flags,  // NOLINT(runtime/int)
-                     const std::string& type);
+                     const std::string& type,
+                     const std::string& fs_mount_options);
 bool UnmountFilesystem(const std::string& mountpoint);
 
 // Returns the block count and the block byte size of the file system on