Remove tabs
nix-copy-closure: Fix --dry-run
nix-copy-closure -s: Do substitutions via ‘nix-store --serve’

This means we no longer need an SSH master connection, since we only
execute a single command on the remote host.
Fix use of sysread
Replace message "importing path <...>" with "exporting path <...>"

This causes nix-copy-closure to show what it's doing before rather
than after.
Fix broken Pid constructor