nixos/ldap: set proper User= and Group= for nslcd service

eb90d970 broke nslcd, as /run/nslcd was
created/chowned as root user, while nslcd wants to do parts as nslcd
user.

This commit changes the nslcd to run with the proper uid/gid from the
start (through User= and Group=), so the RuntimeDirectory has proper
permissions, too.

In some cases, secrets are baked into nslcd's config file during startup
(so we don't want to provide it from the store).

This config file is normally hard-wired to /etc/nslcd.conf, but we don't
want to use PermissionsStartOnly anymore (#56265), and activation
scripts are ugly, so redirect /etc/nslcd.conf to /run/nslcd/nslcd.conf,
which now gets provisioned inside ExecStartPre=.

This change requires the files referenced to in
users.ldap.bind.passwordFile and users.ldap.daemon.rootpwmodpwFile to be
readable by the nslcd user (in the non-nslcd case, this was already the
case for users.ldap.bind.passwordFile)

fixes #57783