chmod 644 for systemd services

Summary:
This fixes the following journal entry:

systemd[1]: Configuration file /usr/lib/systemd/system/lxc-container-mailserver.service
is marked executable. Please remove executable permission bits.
Proceeding anyway.

Fixes T653.

Test Plan: trivial

Reviewers: Kaliiixx

Maniphest Tasks: T653

Differential Revision: http://devcentral.nasqueron.org/D163