Move /src to /subprojects

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

subprojects/libutil/unix/environment-variables.cc

@@ -0,0 +1,22 @@
+#include <cstdlib>
+
+#include "environment-variables.hh"
+
+namespace nix {
+
+int setEnv(const char * name, const char * value)
+{
+    return ::setenv(name, value, 1);
+}
+
+std::optional<std::string> getEnvOs(const std::string & key)
+{
+    return getEnv(key);
+}
+
+int setEnvOs(const OsString & name, const OsString & value)
+{
+    return setEnv(name.c_str(), value.c_str());
+}
+
+}