users: Improve startup of nslcd/slapd

From !2523 (closed), we can improve startup of nslcd with the following:

• Start sshd after nslcd service. Currently, in slow devices, nslcd can start minutes after sshd. This can easily cause user/IP blocking by pam_abl/fail2ban when a user tries 100% correct password multiple times. Some of the reported issues in the Freedombox forum could be related to this kind of blocking.
• Make the nslcd service startup faster. We should investigate whether there are some nslcd and slapd service dependencies that are safe to remove/replace, for example:

After=remote-fs.target
After=network-online.target
Wants=network-online.target