Always have ps output to stdout at least one line.

There is at least one app that assumes that you will always get at least
one line of output in stdout from ps.

To fix this, move error output to stdout, and move the check of whether
/proc can be opened until after the ps header is printed.

Bug: 26554285
Change-Id: I6d9342aafd5c6f728735507cdd87a48a8e0373ac