Adjust --help and manpages to make it easier to find the Web manual.

Adjust --help and manpages to make it easier to find the Web manual.

Today it came to my attention that, if you don’t already know that all
the GNU manuals are readable in HTML format on gnu.org, it’s not easy
to find one starting from typical --help output and/or help2man-
generated manpages.  Let’s do a little to make this easier, and have
“https://www.gnu.org/software/autoconf/manual/” appear prominently in
both the --help output and the trailer text of our generated manpages.

This uses the extended tmac.an “.MT” and “.UR” directives, which are
not universal, but I suspect anyone who’s actively using Autoconf on
an old system does not need the advice, and they can get it from
--help regardless.

* bin/autoconf.in, bin/autoheader.in, bin/autom4te.in, bin/autoreconf.in
* bin/autoscan.in, bin/autoupdate.in, bin/ifnames.in:
  Refer specifically to https://www.gnu.org/software/autoconf/manual/
  for the web version of the complete Autoconf manual.
* man/common.x: Likewise; also, suggest the Savannah tracker as
  well as bug-autoconf for bug reports.