libutil: add setsid support to runProgram2

Change-Id: I9c023f2497cab12f1a7bfafd0baa82ffb5912d3e

lix/libutil/processes.cc

@@ -350,6 +350,9 @@ RunningProgram runProgram2(const RunOptions & options)
     /* Fork. */
     Pid pid{startProcess(
         [&]() {
+            if (options.createSession && setsid() == -1) {
+                throw SysError("setsid");
+            }
             if (options.environment) {
                 replaceEnv(*options.environment);
             }

lix/libutil/processes.hh

@@ -96,6 +96,7 @@ struct RunOptions
     std::optional<std::map<std::string, std::string>> environment = {};
     bool dieWithParent = true;
     bool captureStdout = false;
+    bool createSession = false;
     std::vector<Redirection> redirections;
 #if __linux__
     std::set<long> caps;