\h tab-completion

All,

I'm not really inclined to do it myself right now, but it'd be awful
nice if we had better table completion for \h.

Right now, '\h alter<tab>' returns nothing, and '\h alter' returns a
*bunch* of stuff.

Yet, we happily support '\h alter view' and friends, returning just the
info relevant for that particular command.

This would be a good starter project for someone new to work on, imv,
tho it could also go on the to-do list.

Thanks!

Stephen