library: avoid any --with-systemd conflict, <PIDS> api

library: avoid any --with-systemd conflict, <PIDS> api

A potential conflict arises should should some program
attempt to access systemd data using our library built
without that support (--disable-systemd). And while we
could argue that all callers should check against NULL
string pointers, it's not expected of libprocps users.

So we'll guarantee 'em valid string consisting of "?".

Reference(s):
http://www.freelists.org/post/procps/systemd-binary-vs-library

Signed-off-by: Jim Warner <james.warner@comcast.net>