daemon: let subdaemons outlive their parent again

In 7b37d5ea6a, aside from subdaemons getting
properly executed, they also lost the ability to outlive their parent, due to
now getting set the parent death signal like most other processes spawned by
Lix. This has annoying consequences like all concurrent builds being forcefully
terminated on system updates requiring a nix-daemon restart. As the behaviour
change was not documented and the systemd service file retained
`KillMode=process`, it seems to have been accidental. Restore the old behaviour
of letting the subdaemons outlive their parent.

Change-Id: I6a6a69645312a90dbce55495c2fef3825dd3c097

lix/libutil/processes.cc

@@ -319,7 +319,9 @@ RunningProgram runProgram2(const RunOptions & options)
     Pipe out;
     if (options.captureStdout) out.create();
 
-    ProcessOptions processOptions;
+    ProcessOptions processOptions {
+        .dieWithParent = options.dieWithParent,
+    };
 
     std::optional<Finally<std::function<void()>>> resumeLoggerDefer;
     if (options.isInteractive) {

lix/libutil/processes.hh

@@ -94,6 +94,7 @@ struct RunOptions
     std::optional<uid_t> gid = {};
     std::optional<Path> chdir = {};
     std::optional<std::map<std::string, std::string>> environment = {};
+    bool dieWithParent = true;
     bool captureStdout = false;
     bool isInteractive = false;
     std::vector<Redirection> redirections;

lix/nix/daemon.cc

@@ -358,6 +358,7 @@ try {
                         "--log-level",
                         fmt("%1%", int(verbosity)),
                     },
+                .dieWithParent = false,
                 .redirections =
                     {
                         {.dup = SUBDAEMON_CONNECTION_FD, .from = remote.get()},