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/doc/service-api.txt b/doc/service-api.txt
index 7d30a7e..e0d9b32 100644
--- a/doc/service-api.txt
+++ b/doc/service-api.txt
@@ -1022,6 +1022,31 @@
 			The current value of this property is readable in the
 			"Provider" property of this service.
 
+		int PPPoE.LCPEchoFailure
+
+			(PPPoE only) If non-zero, the number of LCP echo
+			responses that this service will tolerate missing from
+			the PPP peer before terminating the connection.
+
+		int PPPoE.LCPEchoInterval
+
+			(PPPoE only) If non-zero, used as the number of seconds
+			between sending LCP echo requests to the PPP peer.
+
+		string PPPoE.Password
+
+			(PPPoE only) This string is used as the password
+			during authentication with a PPPoE server.  If empty,
+			it will still be used during authentication if
+			requested.
+
+		string PPPoE.Username
+
+			(PPPoE only) If non-empty, this string is used as the
+			username during authentication with a PPPoE server.  If
+			empty, it will still be used during authentication if
+			requested.
+
 		string Passphrase [readwrite]
 
 			If the service type is "wifi", then this property