Lua seg. faults when asked to create the 'activelines' table for a
vararg function with no debug information.

Typos in comments.

Function 'lua_toclose' can raise a non-memory error (but not a
memory error).

Set errno to zero before calling any function where we may use its
errno, and check errno for zero before using it (as functions may
not set it even in error).

ISO C forbids assignment of a union field to another field of the same
union.

Assume that no function will put garbage on errno (although ISO C allows
that). If any function during an operation set errno, and the operation
result in an error, assume that errno has something to say.

Several tools inspect 'lua.h' to extract version information, and
they assume the file will have some specific format.