texinfo should support Emacs coding specifier

[Per Bothner]

There is an Emacs standard for specifying the character encoding of a file,
by putting 'coding: ENCODING' in the mode specifier.
The texinfo specification should follow this standard; it is extra
weird to not do so when the texinfo specification does require a mode 
declaration.

I.e. following file-start should be allowed - and recommended:

\input texinfo.tex      @c -*-texinfo; coding: utf8 -*-

The existing @documentencoding works, but that does not preclude using 
'coding:'.
For one thing: The texinfo manual has a buglet: The @section title is
'Set Input Encoding' when it should be 'Set Input and Output Encoding'.

I suggest semi-deprecating @documentencoding in the sense that a 'coding:'
declaration is preferred and has the same effect.

If a file has both a 'coding:' specifier and @documentencoding, then the
'coding:" specifier applies to the input encoding, and @documentencoding
applies to the *output* encoding only.
--
        --Per Bothner
address@hidden   http://per.bothner.com/