fall back to /proc/<pid>/comm for process name
fall back to /proc/<pid>/comm for process name

when proc/<pid>/cmdline is empty, which is the case for some system
and defunct processes.

Fixes #2315

weaveworks/scope,weaveworks/scope,kinvolk/scope,weaveworks/scope,paulbellamy/scope,alban/scope,kinvolk/scope,weaveworks/scope,alban/scope,alban/scope,paulbellamy/scope,weaveworks/scope,kinvolk/scope,kinvolk/scope,alban/scope,paulbellamy/scope,alban/scope,paulbellamy/scope,kinvolk/scope,kinvolk/scope,weaveworks/scope,paulbellamy/scope,paulbellamy/scope,alban/scope