m_spanningtree Set the bursting flag in the FJOIN handler based on whether the source of the FJOIN is bursting or not

Previously the flag was true if our route to the source was bursting, which resulted in m_joinflood incorrectly registering netjoins when a remote server was bursting

Fixes issue #839 reported by @joshtek0
m_spanningtree Advertise the availability of m_globops in CAPAB CAPABILITIES