Clean up daemon handling

Split `common.sh` into the vars and functions definitions vs starting
the daemon (and possibly other initialization logic). This way,
`init.sh` can just `source` the former. Trying to start the daemon
before `nix.conf` is written will fail because `nix daemon` requires
`--experimental-features 'nix-command'`.

`killDaemon` is idempotent, so it's safe to call when no daemon is
running.

`startDaemon` and `killDaemon` use the PID (which is now exported to
subshells) to decide whether there is work to be done, rather than
`NIX_REMOTE`, which might conceivably be set differently even if a
daemon is running.

`startDaemon` and `killDaemon` can save/restore the old `NIX_REMOTE` as
`NIX_REMOTE_OLD`.

`init.sh` kills daemon before deleting everything (including the daemon
socket).

tests/init.sh

@@ -1,14 +1,13 @@
-set -e -o pipefail
+set -eu -o pipefail
 
-source common.sh
+# Don't start the daemon
+source common/vars-and-functions.sh
 
 test -n "$TEST_ROOT"
 if test -d "$TEST_ROOT"; then
     chmod -R u+w "$TEST_ROOT"
     # We would delete any daemon socket, so let's stop the daemon first.
-    if [[ -n "${NIX_DAEMON_PACKAGE:-}" ]]; then
-        killDaemon
-    fi
+    killDaemon
     rm -rf "$TEST_ROOT"
 fi
 mkdir "$TEST_ROOT"