Warnings with clang when using long double for Lua floats.

More uses of macros 'likely'/'unlikely' (renamed to
'l_likely'/'l_unlikely'), both in range (extended to the
libraries) and in scope (extended to hooks, stack growth).

Corrected several small details: added 'const', adjusts in tabs x
spaces, removed unused #includes and #defines, misspellings, etc.

Several details in code (e.g., moving a variable to the most inner
scope that encloses its uses), comments, parameter names, extra tests.

That macro is applied to float attributes, not to limits.

The macro 'luaL_pushfail' documents all places in the standard libraries
that return nil to signal some kind of failure. It is defined as
'lua_pushnil'. The manual also got a notation (@fail) to document those
returns. The tests were changed to be agnostic regarding whether 'fail'
is 'nil' or 'false'.

A call to 'math.randomseed()' returns the two components of the seed
it set, so that they can be used to set that same seed again.

The previous definition of LUA_MAXUNSIGNED used a typecast,
making it unsuitable for constant expressions in the preprocessor.

- The preprocessor must work with at least 'long', and therefore must
do shifts of up to 31 bits correctly.
- Whenever possible, use unsigned types in shifts.