'lua_resetthread' is back to its original signature, to avoid
incompatibilities in the ABI between releases of the same version.
New function 'lua_closethread' added with the "correct" signature.

Typos in comments and details in the manual.

Also, small tweak in makefile. (-Wsign-compare is already enabled by
-Wextra.)

The size of the list of upvalue names are stored separated from the
size of the list of upvalues, but they share the same array.

Comments in 'onelua.c'

We still need access to a 16-bit system to correctly test
these changes.

It also causes 'L->top' to be wrong when the error happens,
triggering an 'assert'.

There is no need for those intermediate definitions.