fix the syslog reload

[davidcoutadeur]

This issue has been reported by @tchernomax

See: #80

Here are my investigations:

• rsyslog and syslog-ng are both available on Debian and CentOS 7
• rsyslog and syslog-ng are both compatible with OpenLDAP logging
• for now, LTB packages configure automatically rsyslog during install. I see no reason to change this behaviour by default. An admin can then replace rsyslog by syslog-ng
• the proposed MR is not compatible with CentOS, but can be easily adapted.

This problem can be blocking for 2.4, so a backport to 2.4 is desired.

[davidcoutadeur]

Done in 716265e (openldap24) + 72a09bf (openldap25)