shill: expose LCP properties of PPPoE connections

Expose the LCP echo interval and LCP echo failure properties on
PPPoEService instances.  Adds documentation of these new properties to
doc/, as well as the documentation for a few properties that were
overlooked during implementation.

BUG=chrome-os-partner:37468
TEST=FEATURES=test emerge-amd64-generic shill
CQ-DEPEND=CL:257530

Change-Id: I00eccdd75bab46fcd1893cbd9b2351ab65c564f8
Reviewed-on: https://chromium-review.googlesource.com/257570
Reviewed-by: Garret Kelly <gdk@chromium.org>
Tested-by: Garret Kelly <gdk@chromium.org>
Commit-Queue: Garret Kelly <gdk@chromium.org>
diff --git a/ppp_daemon_unittest.cc b/ppp_daemon_unittest.cc
index d066c93..75ee27a 100644
--- a/ppp_daemon_unittest.cc
+++ b/ppp_daemon_unittest.cc
@@ -90,12 +90,15 @@
   options.no_detach = true;
   options.no_default_route = true;
   options.use_peer_dns = true;
+  options.lcp_echo_interval = 1;
+  options.lcp_echo_failure = 1;
 
   Error error;
   std::unique_ptr<ExternalTask> task(Start(options, "eth0", &error));
 
   std::set<std::string> expected_arguments = {
-    "nodetach", "nodefaultroute", "usepeerdns"
+    "nodetach", "nodefaultroute", "usepeerdns", "lcp-echo-interval",
+    "lcp-echo-failure",
   };
   for (const auto &argument : argv_) {
     expected_arguments.erase(argument);