all: use consistent diagnostics for unknown long options

all: use consistent diagnostics for unknown long options

Previously, e.g. cksum failed to output the offending unknown long
option:
  $ cksum --unknown-opt
  cksum: invalid option -- '-'
  Try 'cksum --help' for more information.
i.e., it tried to diagnose '-' as short option.
Instead, it should diagnose the unknown long option:
  $ cksum --unknown-opt
  cksum: unrecognized option '--unknown-opt'
  Try 'cksum --help' for more information.

* src/cksum.c (long_options): Add struct with null entry only.
(main): Use it in the getopt_long call.
* src/dd.c: Likewise.
* src/hostid.c: Likewise.
* src/hostname.c: Likewise.
* src/link.c: Likewise.
* src/logname.c: Likewise.
* src/nohup.c: Likewise.
* src/sleep.c: Likewise.
* src/tsort.c: Likewise.
* src/unlink.c: Likewise.
* src/uptime.c: Likewise.
* src/users.c: Likewise.
* src/whoami.c: Likewise.
* src/yes.c: Likewise.
* NEWS (Improvements): Mention the fix.