PathSubstitutionGoal: Clean up pipe

If there were many top-level goals (which are not destroyed until the
very end), commands like

  $ nix copy --to 'ssh://localhost?remote-store=/tmp/nix' \
    /run/current-system --no-check-sigs --substitute-on-destination

could fail with "Too many open files". So now we do some explicit
cleanup from amDone(). It would be cleaner to separate goals from
their temporary internal state, but that would be a bigger refactor.

src/libutil/util.cc

@@ -752,13 +752,13 @@ AutoCloseFD::AutoCloseFD() : fd{-1} {}
 AutoCloseFD::AutoCloseFD(int fd) : fd{fd} {}
 
 
-AutoCloseFD::AutoCloseFD(AutoCloseFD&& that) : fd{that.fd}
+AutoCloseFD::AutoCloseFD(AutoCloseFD && that) : fd{that.fd}
 {
     that.fd = -1;
 }
 
 
-AutoCloseFD& AutoCloseFD::operator =(AutoCloseFD&& that)
+AutoCloseFD & AutoCloseFD::operator =(AutoCloseFD && that)
 {
     close();
     fd = that.fd;
@@ -789,6 +789,7 @@ void AutoCloseFD::close()
         if (::close(fd) == -1)
             /* This should never happen. */
             throw SysError("closing file descriptor %1%", fd);
+        fd = -1;
     }
 }
 
@@ -822,6 +823,12 @@ void Pipe::create()
 }
 
 
+void Pipe::close()
+{
+    readSide.close();
+    writeSide.close();
+}
+
 
 //////////////////////////////////////////////////////////////////////
 
@@ -1121,7 +1128,7 @@ void runProgram2(const RunOptions & options)
         throw SysError("executing '%1%'", options.program);
     }, processOptions);
 
-    out.writeSide = -1;
+    out.writeSide.close();
 
     std::thread writerThread;
 
@@ -1134,7 +1141,7 @@ void runProgram2(const RunOptions & options)
 
 
     if (source) {
-        in.readSide = -1;
+        in.readSide.close();
         writerThread = std::thread([&]() {
             try {
                 std::vector<char> buf(8 * 1024);
@@ -1151,7 +1158,7 @@ void runProgram2(const RunOptions & options)
             } catch (...) {
                 promise.set_exception(std::current_exception());
             }
-            in.writeSide = -1;
+            in.writeSide.close();
         });
     }