Make libustctl list only online pids v3

Changes since v2:
Botox.

Changes since v1:
Cosmetic surgery at a european cost

Previously libustctl would list all pids. This patch changes this
so only online pids are listed. This is done by appending on each
socket name the mtime from the proc/<pid> directory. This way a
socket can be checked to see if the appended mtime matches the
mtime of the proc dir for the current pid, thus allowing us to
distinguish between old and new socket files.

Signed-off-by: Nils Carlson <nils.carlson@ericsson.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>