This reverts commit 194e3374b8.
Checking the command line for GC roots means that
$ nix-store --delete $path
will fail because $path is now a root because it's mentioned on the
command line.
such as open files, current directories, mmaped files, etc. This is
inherently unportable, but it's easy to adapt this script to other
platforms. Currently we call `lsof' and try to read various bits in
/proc/NNN.
The goal is to prevent the garbage collector from removing store
paths that are no longer reachable from a permanent root but that
are still in use (for instance, after the user has done "nix-env -e"
on a running program).
