shill: add minimum-mtu command-line parameter

Add the --minimum-mtu option to shill to allow clients to override small
MTU values set by DHCP servers.  Any MTU value smaller than the provided
one is dropped.

BUG=chrome-os-partner:36886
TEST=FEATURES=test emerge-amd64-generic shill, ASAN profile

Change-Id: I8e79a18f335705887568e04050712d565d53c4f0
Reviewed-on: https://chromium-review.googlesource.com/263522
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Queue: Garret Kelly <gdk@chromium.org>
Tested-by: Garret Kelly <gdk@chromium.org>
diff --git a/device.h b/device.h
index f66ac28..85a11c4 100644
--- a/device.h
+++ b/device.h
@@ -330,6 +330,7 @@
   FRIEND_TEST(DeviceTest, AvailableIPConfigs);
   FRIEND_TEST(DeviceTest, DestroyIPConfig);
   FRIEND_TEST(DeviceTest, DestroyIPConfigNULL);
+  FRIEND_TEST(DeviceTest, ConfigWithMinimumMTU);
   FRIEND_TEST(DeviceTest, EnableIPv6);
   FRIEND_TEST(DeviceTest, GetProperties);
   FRIEND_TEST(DeviceTest, IPConfigUpdatedFailureWithIPv6Config);