m_spanningtree Introduce new function to send channel messages

Use it from the protocol interface and PRIVMSG/NOTICE handlers
Unite OnUserNotice and OnUserMessage code into LocalMessage()
m_spanningtree Remove encapsulation on UserCount/OperCount
m_spanningtree Change both prefix and first parameter of SQUIT to be a SID
m_spanningtree Netburst: Refactor SendChannelModes()

Rename it to SyncChannel() and change it to take a Channel* parameter, move iteration into DoBurst()
m_spanningtree FJOIN handler: Refactor, update doc

Split the core function into several smaller functions
Accept userlist entries having no comma at all
m_spanningtree Header include cleanup