Fix issue with pid://../run/foo..pid parser

Unfortunately we cannot use realpath(3) here since the the PID files
usually do not yet exist at this point.

Add and modify my ugly de_dotdot() from Merecat httpd.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

TODO.org

@@ -1,4 +1,4 @@
-* TODO fix pid parser, //run/foo.pid -> /var/run///run/foo.pid
+* DONE fix pid parser, //run/foo.pid -> /var/run///run/foo.pid
 ** DONE test framework refactor 'texec sh -c ...' -> 'run ...'
 * TODO Clean up tests a bit
 * TODO Drop tenv or common and rename tenv-root