While we are on it, use '#' symbol as prompt for root-only
commands.

* README.rpm-dist.template (PGSETUP_ROOT_COMMAND): New macro.
(_PGSETUP_COMMAND): New wrapper macro.
(PGSETUP_COMMAND): Use _PGSETUP_COMMAND.
(UPGRADING AN INSTALLATION): s/postgresql/postgresql-setup/.

* configure.ac: Substitute more variables.
* README.rpm-dist.template (Filesystem layout): Fix paths to be
generated correctly even for non-default builds.
(UPGRADING AN INSTALLATION): Fix link to old data directory.

* README.rpm-dist.template: Mainly automatize generation of the
help for 'MULTIPLE POSTMASTERS' scenario.  Fix other sysv/systemd
nuances.
* TODO: Mention some TODOs.

The benefit is that sgml format we write does not need to be kept
formated precisely.  The output format still needs some "style"
fixes, but now docbook2txt is able to generates correct text-file
README.rpm-dist without problems on RHEL 5.10+.

* .gitignore: Added all intermediate files of README file.
* Makefile.am: Generated README file is firstly filtered through
$(SED_RULES), then macros are expanded by m4 and then docbook2txt
is called.
* README: Document subset of dependencies.
* README.rpm-dist.in: Move to ...
* README.rpm-dist.template: ... here, with docbook transformation.
* TODO: Document we should should work docbook docs.
* configure.ac: AC_SUBST also WANT_SYSVINIT.