When there are multiple errors when closing objects, the error
reported by the protected call is the first one, for two reasons:
First, other errors may be caused by this one;
second, the first error is handled in the original execution context,
and therefore has the full traceback.

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.

Closing methods must be run before correcting 'ci->func' when exiting
a vararg function, to get correct debug information (e.g., in case of
errors).

Several small changes from feedback on 5.4 alhpa rc1 (warnings,
typos in the manual, and the like)

- tests show progress in real time, so that we can see maximum
stack levels even if test crashes.
- new test for recursion continuing into message handler.

'luaL_traceback' changed to use an aux buffer instead of concats.
This should reduce the quantity of garbage it generates (in the
form of intermediate strings) while producing a trackback.
It also added information about the number of levels skipped when
skipping levels in a trace.

- new error message for "attempt to assign to const variable"
- note in the manual about compatibility options
- comments
- small changes in 'read_line' and 'pushstr'