gc: improved syntax errors

* example-based syntax errors (go.errors)

* enable bison's more specific errors
  and translate grammar token names into
  tokens like ++

* test cases

R=ken2, r, ken3
CC=golang-dev
https://golang.org/cl/194085
diff --git a/test/run b/test/run
index b9bc5da0..6d9a71f 100755
--- a/test/run
+++ b/test/run
@@ -48,7 +48,7 @@
 
 true >pass.out >times.out
 
-for dir in . ken chan interface nilptr fixedbugs bugs
+for dir in . ken chan interface nilptr syntax fixedbugs bugs
 do
 	echo
 	echo '==' $dir'/'