'luaD_growstack' already checks that. This commit also fixes an
internal bug in 'luaD_growstack': a large 'n' could cause an arithmetic
overflow when computing 'needed'.

- more organized handling of 'nCcalls'
- comments
- deprecation of 'setcstacklimit'

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

The script 'all', to run all tests, automatically ensures that the
Lua interpreter and the test C libraries (in 'testes/libs/') are
updated with any changes in 'luaconf.h'.