A "with stack" implementation gains too little in performance to be
worth all the noise from C-stack overflows.

This commit is almost a sketch, to test performance. There are several
pending stuff:

- review control of C-stack overflow and error messages;
- what to do with setcstacklimit;
- review comments;
- review unroll of Lua calls.

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.