'get_prompt' uses 'luaL_tolstring' to convert _PROMPT or _PROMPT2
value to a string. That conversion may invoke a '__tostring'
metamethod.

To allow their use in memory tests, some functions in 'ltests.c'
should never allocate memory. To avoid this allocation, the
library registers the strings used for status codes, and keeps
the variable '_WARN' always defined (with false instead of nil).

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

When using warn-mode '@store', from the test library, the tests ensure
not only that the expected warnings were issued, but also that there was
no extra warnings.

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.

The test "to-be-closed variables in main chunk" was broken,
as it used the removed feature of functions as to-be-closed values.
The error was not detected because its expected result had no lines
to be checked (due to missing new lines).

Attributes changed to posfixed ('x <const>', instead of '<const> x'),
and "toclose" renamed to "close". Posfixed attributes seem to make it
clearer that it applies to only one variable when there are multiple
variables.

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 flag for to-be-closed variables was changed from '*toclose'
to '<toclose>'. Several people found confusing the old syntax and
the new one has a clear terminator, making it more flexible for
future changes.