Changed the daemons to run as dirsrv by default on Debian/Ubuntu

The previous user/group was nobody:nogroup. While this is in line with what
RedHat suggests (nobody:nobody), it causes issues because the Debian packages
create directories (e.g. /var/log/dirsrv) owned by dirsrv:dirsrv, which causes
access control issues after the ns-slapd has dropped it's root privileges.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Fixed wrong schema file extension in dirsrv::config::schema

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Enhanced documentation

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>