poll: Count failed procedures

When a timeout has occured several times, the procedures handled by
poll_timeout are aborted. This happens when the number of repetitions
exceed N3105. Currently only the timeouts themselves are counted.

This commits adds counters that are incremented if a procedure has
really failed.

New counter:
- rlc.ass.failed:   Count failing UL and DL assigments via PACCH
- rlc.ack.failed:   Count failing DL Ack/Nack requests

Sponsored-by: On-Waves ehf
diff --git a/src/bts.cpp b/src/bts.cpp
index 39737f4..122ff20 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -62,7 +62,9 @@
 	{ "rlc.stalled",		"RLC Stalled          "},
 	{ "rlc.nacked",			"RLC Nacked           "},
 	{ "rlc.ass.timedout",		"RLC Assign Timeout   "},
+	{ "rlc.ass.failed",		"RLC Assign Failed    "},
 	{ "rlc.ack.timedout",		"RLC Ack Timeout      "},
+	{ "rlc.ack.failed",		"RLC Ack Failed       "},
 	{ "rlc.rel.timedout",		"RLC Release Timeout  "},
 	{ "rlc.late-block",		"RLC Late Block       "},
 	{ "decode.errors",		"Decode Errors        "},