Warnings are mostly a tool to help developers (e.g., by showing hidden
error messages); regular users usually don't need to see them.

Added the concept of control messages to the warning system, plus the
implementation of the controls "@on"/"@off" to turn warnings on/off.
Moreover, the warning system in the test library adds some other
controls to ease the test of warnings.

A call to 'math.randomseed()' returns the two components of the seed
it set, so that they can be used to set that same seed again.

The field 'L->nCcalls' now counts downwards, so that the C-stack
limits do not depend on the stack size.

Instead of a 'tocont' flag, the function 'warn' in Lua now receives all
message pieces as multiple arguments in a single call. Besides being
simpler to use, this implementation ensures that Lua code cannot create
unfinished warnings.

- The warning functions get an extra parameter that tells whether
message is to be continued (instead of using end-of-lines as a signal).

- The user data for the warning function is a regular value, instead
of a writable slot inside the Lua state.

Errors in finalizers (__gc metamethods) are never propagated.
Instead, they generate a warning.

The warning system is just a way for Lua to emit warnings, messages
to the programmer that do not interfere with the running program.

  * unification of the 'nny' and 'nCcalls' counters;
  * external C functions ('lua_CFunction') count more "slots" in
    the C stack (to allow for their possible use of buffers)
  * added a new test script specific for C-stack overflows. (Most
    of those tests were already present, but concentrating them
    in a single script easies the task of checking whether
    'LUAI_MAXCCALLS' is adequate in a system.)