Doc: clean up the QProcess::setChildProcessModifier example

- Use nullptr instead of 0
- Pass directory to chroot that is not in /etc
- Set umask to a sensible value (0 is insecure)

Change-Id: I1dba29bc0f454df09ca1fffd161801257f9ccb3c
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/doc/snippets/code/src_corelib_io_qprocess.cpp

@@ -96,12 +96,12 @@ void runSandboxed(const QString &name, const QStringList &arguments)
     proc.setChildProcessModifier([] {
         // Drop all privileges in the child process, and enter
         // a chroot jail.
-        ::setgroups(0, 0);
-        ::chroot("/etc/safe");
+        ::setgroups(0, nullptr);
+        ::chroot("/run/safedir");
         ::chdir("/");
         ::setgid(safeGid);
         ::setuid(safeUid);
-        ::umask(0);
+        ::umask(077);
     });
     proc.start(name, arguments);
     proc.waitForFinished();