lib, zebra: include source client zapi info in opaque messages

lib, zebra: include source client zapi info in opaque messages

Include the sending zapi client info (proto, instance, and
session id) in each opaque zapi message. Add opaque 'init'
apis for clients who want to encode their opaque data inline,
into the zclient's internal stream buffer. Use these init apis
in the TE/link-state lib code, instead of hand-coding the
zapi opaque header info.

Signed-off-by: Mark Stapp <mjs@labn.net>