Open upvalues are kept alive together with their corresponding
stack. This change makes a simpler and safer fix to the issue in
commit 440a5ee78c8, about upvalues in the list of open upvalues
being collected while others are being created. (That previous fix
may not be correct.)

When creating an upvalue, an emergency collection can collect the
previous upvalue where the new one would be linked. The following
code can trigger the bug, using valgrind on Lua compiled with the
-DHARDMEMTESTS option:

  local x; local y
  (function () return y end)();
  (function () return x end)()

New functions to reset/kill a thread/coroutine, mainly (only?) to
close any pending to-be-closed variable. ('lua_resetthread' also
allows a thread to be reused...)

Still missing:
- handling of memory errors when creating upvalue (must run closing
method all the same)
- interaction with coroutines

Version numbers and dates (mostly wrong) from RCS keyword strings
removed from all source files; only the file name are kept.

(we may want to put extra info there in the future)