docs: excorcise NIS from nsswitch.conf

Let's replace the "compat" module in our proposed nsswitch.conf
configuration with "files", since it is not 1995 anymore.

Fedora and other distros have deprecated and removed NIS support a while
back. While others still retain some support I am not sure we should
advertise it in our examples. Downstream can of course still use
"compat" instead of "files" if they want to, but let's not confuse
people who don't care about NIS anymore with this.

Also, bring the nsswitch.conf snippet in README in line with what our
man pages say.

Also see: https://fedoraproject.org/wiki/Changes/retire_NIS_user_space_utils

factory/etc/nsswitch.conf

@@ -1,8 +1,8 @@
 # This file is part of systemd.
 
-passwd:         compat systemd
-group:          compat [SUCCESS=merge] systemd
-shadow:         compat systemd
+passwd:         files systemd
+group:          files [SUCCESS=merge] systemd
+shadow:         files systemd
 gshadow:        files systemd
 
 hosts:          mymachines resolve [!UNAVAIL=return] files myhostname dns