pkill: Return 0 if successfully killed process

Original report:
When trying kill a process with insufficient privileges (see blow),
pkill displays the error message “... failed: Operation not permitted”,
but returns 0. Surely it should return 3?

$ pkill syslogd ; echo $?
pkill: killing pid 373 failed: Operation not permitted
0

Return value 0 means one of more things matched. For a pgrep (which
shares code with pkill) this makes sense, there was a match. It seems
wrong for pkill to return 0 when it in fact could not do what you told
it to.  However return value 3 means a fatal error and it's not fatal.

Looking at other programs when trying to kill things it cannot kill.
shell kill returns 1, procps kill returns 1, killall returns 1, skill
returns 0 (and says it was successful!, ah well poor old skill)

The consensus seems to be that you return 1 if you cannot kill it, even
if you found it. In other words the return value for both not found and
not able to kill it is the same.

pkill only returns 0 if something was killed. This means we found a
match AND the kill() system call worked too.

References:
 https://bugs.debian.org/852758

Signed-off-by: Craig Small <csmall@enc.com.au>
2 jobs for master in 2 minutes and 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #9874602
MakeCheck

00:02:53

failed #9874283
MakeCheck

00:03:12