- Support df command

- Return warnings array from ParseSyntaxFiles() in addition to potential
  error. Not being able to parse the dir is a fatal error but if a
random file cannot be read, it should be considered a warning.

syntax_files/ping.yaml

@@ -24,4 +24,5 @@ rules:
     - red: "DUP\\!"
     - red: "(Destination Host Unreachable|100(\\.0)?% packet loss)"
     - red: ".+unknown\\shost\\s(.+)"
+    - red: "Temporary failure in name resolution"