seneschal: Steward of the user's /home

Add dbus constants and proto definitions for the seneschal service,
which provides fine-grained access to specific paths of the user's home
directory.

BUG=chromium:703939
TEST='emerge-eve system_api'

Change-Id: I0ff0ab5862e1fee6062440891f08424132678371
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1094266
5 files changed