standards.texi update for newer POSIX and C standards

[Paul Eggert]

Here's a proposed patch to standards.texi, to better reflect current 
practice in GNU hacking, and to reflect newer versions of C and POSIX 
standards.

The main change is to say that it's OK to assume C99, except for VLAs. 
Many (most?) GNU programs are doing this already, e.g., by using 
declarations after statements.

This patch also drops coverage of the K&R function definition style, as 
this is no longer needed on practical platforms and C23 no longer 
requires support for that syntax.

Also, C23 removed trigraphs (finally!).

There is no longer a "POSIX.2"; it's just POSIX.1.

Long options like --help are not prohibited by POSIX, as far as I can 
see. Perhaps old POSIX prohibited them, but that ship sailed long ago, 
they're now universal, and I don't see POSIX prohibiting them now.

standards.texi.diff
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