Somebody thought it'd be cute to invent a set of Node tag numbers that were
defined independently of, and indeed conflicting with, the main tag-number
list.  While this accidentally failed to fail so far, it would certainly
lead to trouble as soon as anyone wanted to, say, apply copyObject to these
node types.  Clang was already complaining about the use of makeNode on
these tags, and I think quite rightly so.  Fix by pushing these node
definitions into the mainstream, including putting replnodes.h where it
belongs.

While doing this, also move base backup options into
a struct instead of increasing the number of parameters
to multiple functions for each new option.

This tool makes it possible to do the pg_start_backup/
copy files/pg_stop_backup step in a single command.

There are still some steps to be done before this is a
complete backup solution, such as the ability to stream
the required WAL logs, but it's still usable, and
could do with some buildfarm coverage.

In passing, make the checkpoint request optionally
fast instead of hardcoding it.

Magnus Hagander, reviewed by Fujii Masao and Dimitri Fontaine

Makes it easier to parse mainly the BASE_BACKUP command
with it's options, and avoids having to manually deal
with quoted identifiers in the label (previously broken),
and makes it easier to add new commands and options in
the future.

In passing, refactor the case statement in the walsender
to put each command in it's own function.