kill (-l/-L): print one signal per line when stdout is not a TTY

kill (-l/-L): print one signal per line when stdout is not a TTY

It helps using kill -l/-L in a script.

    $ ./kill -L
     1 HUP      2 INT      3 QUIT     4 ILL      5 TRAP
     6 ABRT     6 IOT      7 BUS      8 FPE      9 KILL
     ...
    $ ./kill -L | cat
    1 HUP
    2 INT
    3 QUIT
    ...

Signed-off-by: Masatake YAMATO <yamato@redhat.com>