Add proto_tree_add_expert and proto_tree_add_expert_format.  This was added to expert.h instead of proto.h because the underlying code to process expert info is static (and should probably remain so).  Also, proto_tree_add_expert and proto_tree_add_expert_format follow "expert info" rules in that they should be called regardless of tree status (even though they take a tree as an argument), unlike the functions in proto.h

Also added an enumeration for checksum validation status, as verifying checksums is considered "expert" functionality.

svn path=/trunk/; revision=50322