Updates to shell portability documentation

Updates to shell portability documentation

* doc/autoconf.texi: Updates all references to "Portable Shell" and
"Limitations of Builtins" to use three-argument commands.
(Programming in M4sh): Document AS_ECHO, AS_ECHO_N, AS_UNSET.
(Portable Shell): Move here discussion about "Where is the POSIX
shell?"  Mention that M4sh provides a SVR2 shell and takes care
of unsetting variables if necessary.  Talk about M4sh and not only
Autoconf-generated scripts.
(Special Shell Variables): Talk about M4sh and not only
Autoconf-generated scripts.  Don't talk about things that Autoconf
does not do.  Mention problems of $LINENO with shell functions.
(Limitations of Builtins).  Mention AS_ECHO and AS_ECHO_N.  Move
discussion of eval bugs before discussion on proper use of eval.
Mention AS_IF.  Reword why not to use "shift N".  Mention "foo=;
unset foo" trick.  Include M4sh code that unsets MAIL for Bash 2.01.
* NEWS: Update list of documented M4sh macros.