* 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.
* gnu/build/install.scm (evaluate-populate-directive): Wrap body in
"catch 'system-error", and report clear errors. In the symlink case,
retry up EEXIST.
(populate-root-file-system): Remove /var/guix/profiles/system-1-link
before attempting to create it.