manual: Convert errno @comments to new @errno macro.

manual: Convert errno @comments to new @errno macro.

errno.texi documents error macros, their values, and error strings in
Texinfo @comments, some of which are also used for @standards.  The
purpose of this commit is to separate the standards from the error
strings so that both the @standards conversion script picks up clean
@standards and the errno documentation framework is improved.

The error names, values, and messages are consolidated in a new custom
macro, @errno.  It is not clear that scripts within the sources rely
on the special Texinfo @comment-based format to generate files used
throughout the library, so the definition of @errno in macros.texi now
provides a comment indicating the dependency.  The dependent scripts
are updated to use @errno, which also simplifies them a bit.  The
files those scripts generate were verified to be unchanged.

The @errno macro is not visibly rendered in any way at this time, but
it does use an @cindex command to add the error string to the Concept
Index, to facilitate searching on error messages.

	* manual/errno.texi: Convert @comment-based errno
	documentation to @errno.
	* manual/macros.texi (@errno): New macro.  Consolidate errors,
	their values, and messages, adding the error string to the
	Concept Index.  Provide a warning in the comment about
	external (to the manual) dependencies.
	* sysdeps/gnu/errlist.awk: Use @errno instead of @comments.
	* sysdeps/mach/hurd/errnos.awk: Likewise.