Add man page-checking Makefile target; make hugetop more Unixy

• man/kill.1: Fix macro usage error
• man/{hugetop,pmap}.1: Alter "EXIT STATUS" layout
• Makefile.am: Add check-man target
• src/hugetop.c: Revise usage message, exit status
• src/hugetop.c: Use (n)curses more idiomatically
• src/hugetop.c: Report errors more idiomatically
• man/hugetop.1: Revise