Change daemon Pidfile class to not use root_helper

Some users of the Pidfile class don't specify root_helper,
which then defaults to 'sudo', which will generate an error.
Remove root_helper altogether since we actually don't need
root priveleges to read /proc/$pid/cmdline.

Changed code to use open.readline() instead of a shell, and
tweaked tests accordingly.

Also cleaned-up the rootwrap filters that allow it as they
are not used anymore.

Fixes bug 1218142

Change-Id: I6691feb1c9f7bfa261a7ec464fd8f3f92168c302
Merge "Change daemon Pidfile class to not use root_helper"