libutil: return pid object from RunningProgram::release

returning the pid as a pid_t instead of as a Pid raii wrapper is only
convenient for daemon use (where it'll eventually go away). using the
released pid correctly in other places is harder without the wrapper.

Change-Id: Ib42a2f357d2f0849beabd015b321bfff31334eda

lix/libutil/processes.cc

@@ -306,9 +306,9 @@ RunningProgram::~RunningProgram()
     }
 }
 
-std::tuple<pid_t, std::unique_ptr<AsyncFdIoStream>> RunningProgram::release()
+std::tuple<Pid, std::unique_ptr<AsyncFdIoStream>> RunningProgram::release()
 {
-    return {pid.release(), std::move(childStdout)};
+    return {std::move(pid), std::move(childStdout)};
 }
 
 int RunningProgram::kill()

lix/libutil/processes.hh

@@ -121,7 +121,7 @@ public:
 
     explicit operator bool() const { return bool(pid); }
 
-    std::tuple<pid_t, std::unique_ptr<AsyncFdIoStream>> release();
+    std::tuple<Pid, std::unique_ptr<AsyncFdIoStream>> release();
 
     int kill();
     [[nodiscard]]

lix/nix/daemon.cc

@@ -363,7 +363,8 @@ try {
                     *forceTrustClientOpt ? "--force-trusted" : "--force-untrusted"
                 );
             }
-            runProgram2(options).release();
+            auto [pid, _stdout] = runProgram2(options).release();
+            pid.release();
         } catch (Error & error) {
             auto ei = error.info();
             // FIXME: add to trace?