Additional cleaning up of Makefile.am for docs

Original nano commit: 4701
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4701
Original nano commit message:
	Simplifying the man rules still further.  Patch by Mike Frysinger.
Only build texinfo docs when 'makeinfo' is available

Original nano commit: 4705
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4705
Original nano commit message:
	Trying to build info only when 'makeinfo' is available.
Remove unused '*_support' variables in configure.ac

Original nano commit: 4708
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4708
Original nano commit message:
	Removing unused '*_support' variables.
Display 'Fatal error: no keys mapped for function ...' message to stderr so it won't be suppressed

Original nano commit: 4717
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4717
Original nano commit message:
	Making sure the user sees a fatal-error message.
Update AUTHORS.nano file with latest AUTHORS file (r4586) from nano repo