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.

- Macro 'checkliveness' (for debug) always uses 'L', to avoid warnings.
- Some old 'while' changed to 'for' in 'testes/gc.lua'.
- In 'testes/libs/makefile', do not make files depend on 'ltests.h',
which may not even exist.

Avoid creating extra strings when possible:

- avoid creating new resulting string when subject was not modified
(instead, return the subject itself);

- avoid creating strings representing the captured substrings when
handling replacements like '%1' (instead, add the substring directly
to the buffer).

- 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.

After a major bad collection (one that collects too few objects),
next collection will be major again. In that case, avoid switching
back to generational mode (as it will have to switch again to
incremental to do next major collection).

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

From the point of view of 'git', all names are relative to the root
directory of the project. So, file names in '$Id:' also should be
relative to that directory: the proper name for test file 'all.lua'
is 'testes/all.lua'.

During generational collection, a userdatum must become gray and
go to a gray list after being traversed (like tables), so that
'correctgraylist' can handle it to its next stage.

This commit also added minimum tests for the generational collector,
including one that would detect this bug.