Also change the default configuration to clear on logout, which is the
upstream default.
* gnu/services/base.scm (<mingetty-configuration>): Add 'clear-on-logout?'
field.
(mingetty-shepherd-service): Pass the "--noclear" option to mingetty only if
'clear-on-logout?' is #false.
* doc/guix.texi (Base Services): Document the 'clear-on-logout?' field.
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