lua: upgrade to Lua 5.1.5.

* src/lua/lbaselib.c, src/lua/ldo.c, src/lua/lgc.c,
src/lua/lparser.c, src/lua/lua.h, src/lua/lua.c: Apply differences
between 5.1.4 and 5.1.5.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
luke: update --version output.

* src/luke.lua (VERSION): Bump to 0.0.1α.
(PACKAGE_BUGREPORT): Point at github issues URL.
* tests/package.lua (AT_PACKAGE_VERSION, AT_PACKAGE_STRING): Bump
to 0.0.1α.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
lua: ensure io.popen is available.

* src/luke.c: Add LUA_USE_POSIX define, to ensure Lua interpreter
implements io.popen.

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