The call to 'checkstackGC' can run finalizers, which will find an
inconsistent CallInfo, as 'ci' is half updated at the point of call.

The pointer to the metamethod can be invalidated by a finalizer that
can run during a GC in 'checkstackGCp'. (This commit also fixes a
detail in the manual.) Bug introduced in commit 91673a8ec.

'coroutine.resume' did not increment counter of C calls when
continuing execution after a protected error (that is,
while running 'precover').

'luaD_pretailcall' mimics 'luaD_precall', handling call metamethods
and calling C functions directly. That makes the code in the
interpreter loop simpler.

This commit also goes back to emulating the tail call in 'luaD_precall'
with a goto, as C compilers may not do proper tail calls and the C
stack can overflow much sooner than the Lua stack (which grows as the
metamethod is added to it).

(plus a detail in lauxlib.h.)

According to ISO C, "making a function an inline function suggests that
calls to the function be as fast as possible." (Not available in C89.)

Not that simpler and slower.

The parameters 'nresults' and 'delta1', in 'luaD_precall', were never
meaningful simultaneously. So, they were combined in a single parameter
'retdel'.

A tail call to a C function can have the behavior of a "real" tail
call, reusing the stack frame of the caller.