shill: Pass a parameter to dhcp client to request a hostname

This will enable dynamic DNS, or at the very least allow DHCP
requests to be identified and distinguished at the server side.

BUG=chromium-os:23741
TEST=New unit tests monitor hostname arg passed to dhcpcd,
and to assure values are being loaded/saved.  Also double
checked on real system by doing a dbus-send to set manager.HostName
and checked that the name was persisted, and that the next
DHCP server we connected to stored this hostname.
CQ-DEPEND=Ic807a2235e0cdcb32a08f9c2c760a86c3579431c

Change-Id: Ic127b65d8563b15e55549a0f029385d0632eaf6c
Reviewed-on: https://gerrit.chromium.org/gerrit/14045
Reviewed-by: Paul Stewart <pstew@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Paul Stewart <pstew@chromium.org>
diff --git a/default_profile.h b/default_profile.h
index 35fb5e4..7e0f5cf 100644
--- a/default_profile.h
+++ b/default_profile.h
@@ -55,6 +55,7 @@
   static const char kDefaultId[];
   static const char kStorageId[];
   static const char kStorageCheckPortalList[];
+  static const char kStorageHostName[];
   static const char kStorageName[];
   static const char kStorageOfflineMode[];