separate initdb -A options for local and host

I think it would be useful to have separate initdb -A options for local
and host entries.  In 9.1, we went out of our way to separate the "peer"
and "ident" methods, but we have moved the confusion into the initdb -A
option, where "ident" sometimes means "peer", and "peer" sometimes means
"ident".  Moreover, having separate options would allow what I think
would be a far more common use case, namely having local "peer" and host
something other than "ident", such as "md5".

I'm thinking, we could keep the existing -A option, but add long options
such as --auth-local and --auth-host, to specify more detail.