Create /run/setuid-programs compatibility symlinks so that we can
migrate all users (both package and human) piecemeal at our leisure.
Apart from being symlinks, this should be a user-invisible change.
* gnu/build/activation.scm (%privileged-program-directory): New variable.
[activate-setuid-programs]: Put privileged copies in
%PRIVILEGED-PROGRAM-DIRECTORY, with compatibility symlinks to each in
%SETUID-DIRECTORY.
* gnu/services.scm (setuid-program-service-type): Update docstring.
* doc/guix.texi (Setuid Programs): Update @file{} name accordingly.
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