'simplesect' encloses the introductory text of sections with
subsections, so that each section either is all text or is all
subsections. (This commit also corrects a small brace error in the
manual and extra spaces/tabs in some other files.)

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'.

- 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.

Added '#if !defined' in some definitions to allow external definitions;
more comments; other small changes.

It is not a good idea to use negative words to describe boolean
values. (When we negate that boolean we create a double negative...)

All UTF-8 encoding functionality (including the escape
sequence '\u') accepts all values from the original UTF-8
specification (with sequences of up to six bytes).

By default, the decoding functions in the UTF-8 library do not
accept invalid Unicode code points, such as surrogates. A new
parameter 'nonstrict' makes them accept all code points up to
(2^31)-1, as in the original UTF-8 specification.

An 'int' may have only 16 bits, so it may not be big enough for UTF-8
values. The new type 'utfint' (in the utf8 library) ensures at least
21 bits for those values.

that the function cannot continue past that call)