C tail calls

[gottfried leibniz <gottfried.leibniz.windoze@...>]

Hello,

Should luaD_precall's invocation of luaD_hook, see [1], reflect the
recent changes to C functions and tail calls (i.e., check CIST_TAIL)?

For instance

      function b() return string.format("%s %s!", "Hello", "World") end
      function a() return b() end
      local result = a()

Will generate debug events: "call", "tail call", "call", "return".
Compared previously: "call", "tail call", "call", "return", "return".

Thanks.

[1] https://github.com/lua/lua/blob/master/ldo.c#L538