Turns out we need to support upgrade from more than one previous
database versions.  The plan is to make this as general as easily
possible, "run-time" configurable without need to recompile.

* postgresql-setup.in: Add options --upgrade-from and
--upgrade-ids.  Require root access (as late as possible).
($USAGE_STRING): Adjust help output accordingly.
($PGMAJORVERSION, $PREVMAJORVERSION)
($PREVPGENGINE): Remove newly useless globals.
($option_upgradefrom): New option default.
(set_var, root_prereq): New helper functions.
(parse_upgrade_setup): Helper function to parse upgrade
configuration.
(print_version): Don't print PREVMAJORVERSION as it is (with
configuration) useless.
(upgrade): Create new $inplace local to handle in-place and
copy-like migrations differently.  Use the variable set generated
by parse_upgrade_setup() function.  Small err message fixes.
(handle_service_env, handle_pgconf): Remove the MODE argument.
(parse_configuration): Generalize operations which needed to be
done on two places.
* upgrade_config.template: Example of ./configure time config
file.  Based on files like this the run-time is generated.
* share/postgresql-setup/.gitignore: New file.
* share/postgresql-setup/Makefile.inc: New file.
* build-helpers/build-config: New file, generator of configuration
files.
* build-helpers/Makefile.inc: New file.
* Makefile.am: Distribute upgrade_config.template.  Start the
conversion to flat Makefile and include two new Makefile.inc
snippets.
* configure.ac: New --with-upgrade-config option.  Propagate the
rawpkgdatadir and UPGRADE_CONFIG_LIST variables into Makefile.
(INSTANTIATE): Enhance to create the leading path also.
(INSTANTIATE_CONV): Convenience helper for leading path of target
creation.
* auxdir/install_local: New (just-for-git) installation helper.
* .gitignore: Don't hide share/postgresql-setup.
* TODO: Mention that KDE folks would like to migrate somehow.