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.

Since commit ca6fe7449a74, userdata with uservalues can be gray
and can belong to gray lists ('gclist').

Due to emergency collections, 'luaE_shrinkCI' can be called while Lua
is building a new CallInfo, which for a while is still a free CallInfo.

Avoid undefined behavior in calls like «fprintf("%s", NULL)».
('lua_writestringerror' is implemented as 'fprintf', and 'lua_tostring'
can return NULL if object is not a string.)

The parser were mixing compiler indices of variables with stack indices,
so that when a to-be-closed variable was used inside the scope of
compile-time constants (which may be optimized away), it might be closed
in the wrong place. (See new tests for examples.)

Besides fixing the bugs, this commit also changed comments and variable
names to avoid that kind of confusion and added tests.

- more consistent nomenclature for error handling
- more precise definition for dead objects
- added algorithm used by 'math.random'
- added luaL_pushfail
- some other minor changes