Fix cosmetic problems in YAML source

This change corrects problems reported by the `yamllint` linter.

Since key duplication problems were removed in 4d48711, this commit
mainly fixes trailing spaces and extra empty lines at beginning/end of
files.

test/samples/test_pb.yml

@@ -39,4 +39,3 @@
     debug: msg="this is the foo handler"
   - name: bar
     debug: msg="this is the bar handler, you should not see this"
-