Update Makefile to correctly set file permissions

[nikosmeds]

Set example config and systemd unit file permissions to 0644, which matches their values in the examples/ directory.

This is required because install command sets all files to 0755 by default:

man install
[...]
     -m      Specify an alternate mode.  The default mode is set to rwxr-xr-x (0755).  The specified mode may be either
             an octal or symbolic value; see chmod(1) for a description of possible mode values.

Alternatively, we could opt to use cp, but the result would be the same.

Closes #440.

[trotttrotttrott]

LGTM

[Dieterbe]

thanks to both of you 😍