fix #2309

[slingamn]

If bob is monitoring alice, bob should get METADATA lines for alice even if bob doesn't have extended-monitor. The four caps that explicitly require extended-monitor are: away-notify, chghost, setname, account-notify.

[slingamn]

We might not even need to check extended-monitor --- we need to advertise it to tell clients that we support it, but there's no real harm in sending, e.g. CHGHOST to a client that requested chghost but not extended-monitor. If they're not tracking the target, they'll just ignore it.