Add support for cap account-tag

The account-tag capability is now requested when ZNC connects to an IRC server, the same is then offered to clients connecting.
This permits the tag to then pass through to account-tag aware clients.

src/IRCSock.cpp

@@ -67,6 +67,7 @@ CIRCSock::CIRCSock(CIRCNetwork* pNetwork)
       m_bUHNames(false),
       m_bAwayNotify(false),
       m_bAccountNotify(false),
+      m_bAccountTag(false),
       m_bExtendedJoin(false),
       m_bServerTime(false),
       m_sPerms("*!@%+"),
@@ -376,6 +377,7 @@ bool CIRCSock::OnCapabilityMessage(CMessage& Message) {
             {"userhost-in-names", [this](bool bVal) { m_bUHNames = bVal; }},
             {"away-notify", [this](bool bVal) { m_bAwayNotify = bVal; }},
             {"account-notify", [this](bool bVal) { m_bAccountNotify = bVal; }},
+            {"account-tag", [this](bool bVal) { m_bAccountTag = bVal; }},
             {"extended-join", [this](bool bVal) { m_bExtendedJoin = bVal; }},
             {"server-time", [this](bool bVal) { m_bServerTime = bVal; }},
             {"znc.in/server-time-iso",