Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/unix-domain-socket.hh

@@ -0,0 +1,83 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "file-descriptor.hh"
+
+#ifdef _WIN32
+#  include <winsock2.h>
+#endif
+#include <unistd.h>
+
+namespace nix {
+
+/**
+ * Create a Unix domain socket.
+ */
+AutoCloseFD createUnixDomainSocket();
+
+/**
+ * Create a Unix domain socket in listen mode.
+ */
+AutoCloseFD createUnixDomainSocket(const Path & path, mode_t mode);
+
+/**
+ * Often we want to use `Descriptor`, but Windows makes a slightly
+ * stronger file descriptor vs socket distinction, at least at the level
+ * of C types.
+ */
+using Socket =
+#ifdef _WIN32
+    SOCKET
+#else
+    int
+#endif
+    ;
+
+#ifdef _WIN32
+/**
+ * Windows gives this a different name
+ */
+#  define SHUT_WR SD_SEND
+#  define SHUT_RDWR SD_BOTH
+#endif
+
+/**
+ * Convert a `Socket` to a `Descriptor`
+ *
+ * This is a no-op except on Windows.
+ */
+static inline Socket toSocket(Descriptor fd)
+{
+#ifdef _WIN32
+    return reinterpret_cast<Socket>(fd);
+#else
+    return fd;
+#endif
+}
+
+/**
+ * Convert a `Socket` to a `Descriptor`
+ *
+ * This is a no-op except on Windows.
+ */
+static inline Descriptor fromSocket(Socket fd)
+{
+#ifdef _WIN32
+    return reinterpret_cast<Descriptor>(fd);
+#else
+    return fd;
+#endif
+}
+
+/**
+ * Bind a Unix domain socket to a path.
+ */
+void bind(Socket fd, const std::string & path);
+
+/**
+ * Connect to a Unix domain socket.
+ */
+void connect(Socket fd, const std::string & path);
+
+}