Fix support for /msg @#chan :hi

005 STATUSMSG defines list of characters prependable to channel name,
but we used simple modes instead.

See
https://tools.ietf.org/html/draft-brocklesby-irc-isupport-03#section-3.16

Fix #272

Thanks to grawity for the link to 005 docs draft, and to carrot for
testing the patch.

include/znc/IRCSock.h

@@ -102,6 +102,7 @@ public:
 	bool IsAuthed() const { return m_bAuthed; }
 	bool IsCapAccepted(const CString& sCap) { return 1 == m_ssAcceptedCaps.count(sCap); }
 	const MCString& GetISupport() const { return m_mISupport; }
+	CString GetISupport(const CString& sKey, const CString& sDefault = "") const;
 	// !Getters
 
 	// This handles NAMESX and UHNAMES in a raw 353 reply