Errors in finalizers need a valid 'pc' to produce an error message,
even if the error is not propagated. Therefore, calls to the GC (which
may call finalizers) inside luaV_execute must save the 'pc'.

Macro 'checkstackGC' was doing a GC step after resizing the stack;
the GC could shrink the stack and undo the resize.

Macro 'checkstackp' also does a GC step, which could remove the
preallocated CallInfo when calling a function.

In 'lundump.c', when loading the upvalues of a function, there can be
a read error if the chunk is truncated. In that case, the creation
of the error message can trigger an emergency collection while the
prototype is still anchored. So, the prototype must be GC consistent
before loading the upvales, which implies that it the 'name' fields
must be filled with NULL before the reading.

When an object aged OLD1 is finalized, it is moved from the list
'finobj' to the *beginning* of the list 'allgc'. So, this part of the
list (and not only the survival list) must be visited by 'markold'.

Added as incompatibility, in the manual, the extra return of 'io.lines'.

'lua_resetthread' should reset the CallInfo list before calling
'luaF_close'. luaF_close can call functions, and those functions
should not run with dead functions still in the CallInfo list.

While building a new prototype, the GC needs barriers for every object
(strings and nested prototypes) that is attached to the new prototype.