Make vtysh work on NetBSD, and minor cleanups:

Make vtysh work on NetBSD, and minor cleanups:

  if --enable-vtysh, look for perl, and substitute into vtysh/extract.pl,
  rather than assuming perl is in /usr/bin

  Look for tputs in libtermcap and libcurses, in addition to
  previously-searched locations.  Follow GNU readlines search order.

  Clean up --enable-vtysh definition in configure.ac.

  Add vtysh/vtysh_cmds.c to CLEANFILES.  This is important if
  extract.pl fails, so that 'make clean && make' will regenerate
  vtysh_cmds.c rhather than using the old zero-length file.