kill: add a feature decoding signal masks

kill: add a feature decoding signal masks

-l/--list option is extended to work well with the output of ps command:

   $ ps s $$
     UID     PID          PENDING          BLOCKED          IGNORED           CAUGHT STAT TTY        TIME COMMAND
    1000 1588189 0000000000000000 0000000000010000 0000000000384004 000000004b813efb S    pts/56     0:00 bash

   $ ./kill --list 0x000000004b813efb
   HUP
   INT
   ILL
   TRAP
   ABRT
   ...

If you know the pid you are interested in, you can skip running ps
comman with -d/--show-process-state option:

   $ ./kill --show-process-state $$
   Blocked: INT
   Ignored: TERM TSTP TTIN TTOU
   Caught: HUP INT PIPE ALRM CHLD WINCH

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