userdbd: reverse which path is a socket and which a symlink

userdbd listens on "two" sockets, that are actually the same: one is a
real AF_UNIX socket in the fs, and the other is a symlink to it.

So far, when userdbd was started from the command line it would make one
a symlink and the other a real socket, but when invoked via unit files
they'd be swapped, i.e. the other would be a symlink and the one a real
socket.

Let's bring this in line.

Since the "io.systemd.Multiplexer" is our main interface, let's make it
the one exposed as socket, and then make "io.systemd.NameServiceSwitch"
a symlink to it. Or in other words, let's adjust the C code to match the
unit file.