Fixes <http://bugs.gnu.org/18525>.
Reported by Mark H Weaver <mhw@netris.org>.
* gnu/build/activation.scm (activate-modprobe): New procedure.
* gnu/system.scm (modprobe-wrapper): New procedure.
(operating-system-activation-script): Use both.
* gnu/build/activation.scm (activate-current-system): Honor
$GUIX_NEW_SYSTEM by default.
* guix/scripts/system.scm (switch-to-system): Set $GUIX_NEW_SYSTEM
before loading SCRIPT.
* gnu/build/install.scm (directives): Remove "/bin/sh".
* gnu/build/activation.scm (activate-/bin/sh): New procedure.
* gnu/system.scm (operating-system-activation-script): Use it.
Before that, 'sudo' would exit with:
sudo: /etc/sudoers is not a regular file
sudo: no valid sudoers sources found, quitting
* gnu/build/activation.scm (activate-etc): Check if SOURCE matches
'file-is-directory?'. If not, use 'copy-file' instead of 'symlink'.