In the chroot, make all mounted filesystems private

This is required on systemd, which mounts filesystems as "shared"
subtrees.  Changes to shared trees in a private mount namespace are
propagated to the outside world, which is bad.

src/libutil/util.cc

@@ -224,12 +224,12 @@ string readFile(int fd)
 }
 
 
-string readFile(const Path & path)
+string readFile(const Path & path, bool drain)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY);
     if (fd == -1)
         throw SysError(format("opening file `%1%'") % path);
-    return readFile(fd);
+    return drain ? drainFD(fd) : readFile(fd);
 }
 
 

src/libutil/util.hh

@@ -63,7 +63,7 @@ Strings readDirectory(const Path & path);
 
 /* Read the contents of a file into a string. */
 string readFile(int fd);
-string readFile(const Path & path);
+string readFile(const Path & path, bool drain = false);
 
 /* Write a string to a file. */
 void writeFile(const Path & path, const string & s);