nixos/memcached: make unix sockets usuable

before:
  - /var/run/memcached is a bad default for a socket path, since its
    parent directory must be writeable by memcached.
  - Socket directory was not created by the module itself -> this was
    left as a burden to the user?
  - Having a static uid with a dynamic user name is not very useful.

after:
  - Replace services.memcached.socket by a boolean flag. This simplifies
    our code, since we do not have to check if the user specifies a
    path with a parent directory that should be owned by memcached
    (/run/memcached/memcached.sock -> /run/memcached).
  - Remove fixed uid/gid allocation. The only file ever owned by the
    daemon is the socket that will be recreated on every start.
    Therefore user and group ids do not need to be static.
  - only create the memcached user, if the user has not specified a
    different one. The major use case for changing option is to allow
    existing services (such as php-fpm) opening the local unix socket.
    If we would unconditionally create a user that option would be
    useless.