libutil/runProgram2: add explicit argv0 support

Change-Id: I292aed7f25de1c193f6e2374c1f6a7ba9d272dd4

lix/libutil/processes.cc

@@ -401,7 +401,7 @@ RunningProgram runProgram2(const RunOptions & options)
 #endif
 
         Strings args_(options.args);
-        args_.push_front(options.program);
+        args_.push_front(options.argv0.value_or(options.program));
 
         restoreProcessContext();
 

lix/libutil/processes.hh

@@ -83,6 +83,7 @@ struct RunOptions
 
     Path program;
     bool searchPath = true;
+    std::optional<std::string> argv0;
     Strings args = {};
     std::optional<uid_t> uid = {};
     std::optional<uid_t> gid = {};