Valgrind: Add an automated test

This makes nginx run in the background under valgrind,
with both a master and a child process.
Valgrind errors will be redirected to `valgrind.log`.
When `USE_VALGRIND` is set, all system tests will be run under valgrind,
and at the end a new test is appended which ensures no valgrind errors
where encountered.

It is also worth noting that:
- There is a new file, `valgrind.sup`, which contains a few suppressions.
- Some tests behave flakey under valgrind. For now these are appended
  to the expected failures (when under valgrind only).
- 'Possibly lost' errors are all suppressed to get the amount of false
  positives manageable.