Move /src to /subprojects

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

subprojects/libutil/users.hh

@@ -0,0 +1,70 @@
+#pragma once
+///@file
+
+#include "types.hh"
+
+#ifndef _WIN32
+# include <sys/types.h>
+#endif
+
+namespace nix {
+
+std::string getUserName();
+
+#ifndef _WIN32
+/**
+ * @return the given user's home directory from /etc/passwd.
+ */
+Path getHomeOf(uid_t userId);
+#endif
+
+/**
+ * @return $HOME or the user's home directory from /etc/passwd.
+ */
+Path getHome();
+
+/**
+ * @return $NIX_CACHE_HOME or $XDG_CACHE_HOME/nix or $HOME/.cache/nix.
+ */
+Path getCacheDir();
+
+/**
+ * @return $NIX_CONFIG_HOME or $XDG_CONFIG_HOME/nix or $HOME/.config/nix.
+ */
+Path getConfigDir();
+
+/**
+ * @return the directories to search for user configuration files
+ */
+std::vector<Path> getConfigDirs();
+
+/**
+ * @return $NIX_DATA_HOME or $XDG_DATA_HOME/nix or $HOME/.local/share/nix.
+ */
+Path getDataDir();
+
+/**
+ * @return $NIX_STATE_HOME or $XDG_STATE_HOME/nix or $HOME/.local/state/nix.
+ */
+Path getStateDir();
+
+/**
+ * Create the Nix state directory and return the path to it.
+ */
+Path createNixStateDir();
+
+/**
+ * Perform tilde expansion on a path, replacing tilde with the user's
+ * home directory.
+ */
+std::string expandTilde(std::string_view path);
+
+
+/**
+ * Is the current user UID 0 on Unix?
+ *
+ * Currently always false on Windows, but that may change.
+ */
+bool isRootUser();
+
+}