Corrections in the implementation of '%' for floats.

The multiplication (m*b) used to test whether 'm' is non-zero and
'm' and 'b' have different signs can underflow for very small numbers,
giving a wrong result. The use of explicit comparisons solves this
problem. This commit also adds several new tests for '%' (both for
floats and for integers) to exercise more corner cases, such as
very large and very small values.

lvm.h

@@ -116,6 +116,7 @@ LUAI_FUNC void luaV_execute (lua_State *L, CallInfo *ci);
 LUAI_FUNC void luaV_concat (lua_State *L, int total);
 LUAI_FUNC lua_Integer luaV_div (lua_State *L, lua_Integer x, lua_Integer y);
 LUAI_FUNC lua_Integer luaV_mod (lua_State *L, lua_Integer x, lua_Integer y);
+LUAI_FUNC lua_Number luaV_modf (lua_State *L, lua_Number x, lua_Number y);
 LUAI_FUNC lua_Integer luaV_shiftl (lua_Integer x, lua_Integer y);
 LUAI_FUNC void luaV_objlen (lua_State *L, StkId ra, const TValue *rb);