Improve coverage of ‘echo’ in manual

Improve coverage of ‘echo’ in manual

In manual, prefer AS_ECHO or printf instead of echo,
when that might make a difference.
Don’t use ‘date >timestamp’, to avoid spurious diffs.
* doc/autoconf.texi (Notices): No need to worry any more about
RCS or CVS being used to maintain Autoconf.
(Preset Output Variables): Say that ECHO_N etc. are obsolete.
(Automatic Remaking): Remove obsolescent mention of CVS.
(AS_ECHO): Mention that AS_ECHO expands to something involving “'”,
and how to work around this (rare) issue.
(Shell Substitutions, Slashes, Limitations of Builtins): Omit
no-longer-relevant discussion about ancient shell bugs that
involved a lot of ‘echo’s that would otherwise need to be
replaced.
(Limitations of Builtins): Don’t mention ‘echo "x$word"’ trick, as
it doesn’t resist backslashes.  Just use printf.  Expand on
discussion of ‘echo’ options.  Say that even modernish shells (e.g.,
ksh93 on OmniOS) sometimes screw up with here-documents.
(Making testsuite Scripts): Simplify $(srcdir)/package.m4
rule by using printf instead of echo, and defend against
some shell metacharacters in $(srcdir).