Fix shutdown behavior and resource management for recursive-nix on macOS

Previously, we relied on the `shutdown()` function to terminate `accept()`
calls on a listening socket. However, this approach did not work on macOS as
the waiting `accept()` call is not considered a connected socket, resulting in
an `ENOTCONN` error. Instead, we now close the listening socket to terminate
the `accept()` call.

Additionally, we fixed a resource management issue where we set the
`daemonSocket` variable to -1, triggering resource cleanup and causing the
`stopDaemon` function to be called twice. This resulted in errors as the socket
was already closed by the time the second `stopDaemon` call was made. Instead of
setting `daemonSocket` to -1, we now release the socket using the `release()`
method on a unique pointer. This properly transfers ownership and allows for
correct resource cleanup.

These changes ensure proper behavior and resource management for the
recursive-nix feature on macOS.

tests/recursive.sh

@@ -3,9 +3,6 @@ source common.sh
 sed -i 's/experimental-features .*/& recursive-nix/' "$NIX_CONF_DIR"/nix.conf
 restartDaemon
 
-# FIXME
-if [[ $(uname) != Linux ]]; then skipTest "Not running Linux"; fi
-
 clearStore
 
 rm -f $TEST_ROOT/result