monitor: Use the systemd Unix socket

monitor: Use the systemd Unix socket

Systemd creates an Unix socket when the system is booted on
by using existing configuration files in /usr/lib/systemd.

Previously, the resource monitor always creates a new Unix
socket whether the socket exists or not. From now on, the
resource monitor checks that the Unix socket is existed on
the given path. If so, it uses the Unix socket instead of
open a newer one. By doing so, the permission of the Unix
socket can follow the given configuration denoted in the
Systemd socket configuration file.

Change-Id: I1eab46cd238340cc9700e1a2d52481270c06b8b1
Signed-off-by: Sung-hun Kim <sfoon.kim@samsung.com>