lyaml: use STREQ/STRNEQ instead of !strcmp/strcmp.

* lyaml.c (load_scalar, lua_yaml_dumper, table_type)
(handle_config_options): Use GNUish STREQ/STRNEQ macros for
improved readability over reverse logic strcmp.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: remove spurious trailing whitespace.

* README.md, bootstrap.conf, configure.ac, lyaml.c: Remove
spurious trailing whitespace.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: move to self-managed slingshot.

* slingshot: Add as a git submodule.
* bootstrap.slingshot: New file, copied from slingshot.
* Makefile.am: Move from here...
* local.mk: ...to here, and adjust for slingshot.
* GNUmakefile, build-aux/do-release-commit-and-tag,
build-aux/gitlog-to-changelog, build-aux/mkrockspecs,
build-aux/release.mk, build-aux/rockspecs.mk, m4/ax_lua.m4,
m4/ax_compare_version.m4: Delete.  Slingshot takes care of these
files automatically.
* .gitignore: Update.
* .travis.yml: Regenerate.

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