BUG/MEDIUM: mworker: Fix re-exec when haproxy is started from PATH

BUG/MEDIUM: mworker: Fix re-exec when haproxy is started from PATH

If haproxy is started using the name of the binary only (i.e.
not using a relative or absolute path) the `execv` in
`mworker_reload` fails with `ENOENT`, because it does not
examine the `PATH`:

  [WARNING] 315/161139 (7) : Reexecuting Master process
  [WARNING] 315/161139 (7) : Cannot allocate memory
  [WARNING] 315/161139 (7) : Failed to reexecute the master processs [7]

The error messages are misleading, because the return value of
`execv` is not checked. This should be fixed in a separate commit.

Once this happened the master process ignores any further
signals sent by the administrator.

Replace `execv` with `execvp` to establish the expected
behaviour.

This bug was introduced in commit 73b85e75b3963086be889e1fb40a59e7ef2ad63b.