maint: prevent pointer type mismatch warnings.

* ext/yaml/lyaml.h (lua_pushyamlstr): Cast away the impedence
mismatch between Lua API 'unsigned char *' strings and libYAML
API 'yaml_char_t *' strings.
(RAWSET_BOOLEAN, RAWSET_INTEGER, RAWSET_STRING, RAWSET_EVENTF)
(RAWGETS_STRDUP): Use lua_pushyamlstr to avoid compiler warnings.
* ext/yaml/parser.c (Pparser): Cast away type differences.
* ext/yaml/scanner.c (Pscanner): Likewise.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
slingshot: sync from upstream, and simplify accordingly.

* slingshot: Sync with upstream.
* bootstrap: Update from slingshot.
* bootstrap.slingshot: Remove. No longer required.
* bootstrap.conf: Remove bootstrap slingshot source boilerplate.
(func_ensure_changelog): Remove.  Automated by slingshot now.
* configure.ac (AM_INIT_AUTOMAKE): Remove 'foreign'.
* COPYING: New file, with MIT License, to avoid Autotools
copying over a standard GPLv3 COPYING boilerplate.
* .travis.yml: Regenerate.

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