References: https://sysctl-explorer.net/fs/protected_hardlinks/ https://sysctl-explorer.net/fs/protected_symlinks/ * gnu/services/sysctl.scm (%default-sysctl-settings): New public variable. (<sysctl-configuration>): Use %default-sysctl-settings as the default value. * gnu/services/base.scm (%base-services): Add sysctl-service-type. * doc/guix.texi (Miscellaneous Services): Document the new defaults. Signed-off-by: Ludovic Courtès <ludo@gnu.org>