pidof: check cmd if space in argv0

A difference in behaviour between the sysvinit and procps pidof
was that the procps one would sometimes not find process that
the sysvinit one did.

The difference is that if a space is found in argv[0] then sysvinit
would look at cmd for a match. This isn't perfect and more of a
best guess but does often work.

procps pidof now follows the same "standard". The most obvious
difference is with kde based processes and incoming ssh connections
with sshd.

References:
 GitLab issue #4
 procps-ng/procps#4
 https://github.com/limingth/sysvinit/blob/master/sysvinit-2.88dsf/src/killall5.c#L800