bootstrap: auto-merge bootstrap.slingshot into bootstrap.

* src/merge-sections.in: New file. Merge sections of named files
into input from stdin.
* src/src.mk (EXTRA_DIST): Add build-aux/merge-sections.
* configure.ac (AC_CONFIG_FILES): Generate build-aux/merge-sections
from src/merge-sections.in.
* bootstrap, bootstrap.slingshot: Move from here...
* bulid-aux/bootstrap.in, bulid-aux/bootstrap.slingshot: ...to here.
* .gitignore: Adjust.
* build-aux/release.mk (release_extra_dist): Remove
bootstrap.slingshot.
* local.mk (EXTRA_DIST): Add build-aux/bootstrap.slingshot and
build-aux/bootstrap.
* bootstrap.conf (require_bootstrap_uptodate): Override bootstrap
default with a custom version that knows how to merge in slingshot
specific functions to upstream bootstrap.
* README.md: Remove bootstrap.slingshot instructions.
* bootstrap: Regenerate.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Release version 6

* NEWS: Record release date.
maint: post-release administrivia.

* NEWS: Add header line for next release.
* .prev-version: Record previous version.
* ./local.mk (old_NEWS_hash): Auto-update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>