OpenFlow: Improve the common header dissection.

OpenFlow: Improve the common header dissection.

of_header_print() printed any version as "unknown", always printed the
type in hex only and printed the length with no remarks. If the header
was not good enough to proceed, openflow_print() would just print
"(invalid)" after it and be done with it.

Introduce of10_msgtype_str(). Rename of10_header_body_print() to
of10_message_print() and unburden it of all common header details. Add a
registry of all known OpenFlow version numbers. Merge of_header_print()
into openflow_print() and take the header apart one field at a time with
the captured and the declared lengths in mind, and indicate any anomalies
in version, type and length independently. Try to print the transaction
ID too before giving up on the header.

Among other changes, the header now shows the type as "unknown" when the
decoder does not know which types are valid for the version, and as
"invalid" when the type is definitely not valid for the version. Update
two tests.