Reject GWT-RPC requests where the serialization policy name contains special characters.

This is to avoid potential security issues in subtypes of SerializationPolicyProvider that use the strongName to do I/O without checking it.

(This is unlikely to be a real problem because serialization policy providers typically append ".gwt.rpc" to create the filename and the names of serialization policy files are hard to guess.)

Review by: mdempsky@google.com
Fix NullPointerException when the filename of a JDT Problem is null.

Review by: mdempsky@google.com
Skip strong name validation when no serialization policy provider is set.

Review by: mdempsky@google.com
Check the result of mkdir when creating a directory.

Review by: mdempsky@google.com