Refactoring: Move all fork handling into a higher-order function

C++11 lambdas ftw.

src/libutil/util.hh

@@ -7,6 +7,7 @@
 #include <dirent.h>
 #include <unistd.h>
 #include <signal.h>
+#include <functional>
 
 #include <cstdio>
 
@@ -237,6 +238,7 @@ class Pid
     int killSignal;
 public:
     Pid();
+    Pid(pid_t pid);
     ~Pid();
     void operator =(pid_t pid);
     operator pid_t();
@@ -252,6 +254,11 @@ public:
 void killUser(uid_t uid);
 
 
+/* Fork a process that runs the given function, and return the child
+   pid to the caller. */
+pid_t startProcess(std::function<void()> fun, const string & errorPrefix = "error: ");
+
+
 /* Run a program and return its stdout in a string (i.e., like the
    shell backtick operator). */
 string runProgram(Path program, bool searchPath = false,