mpm_winnt: Remove an unnecessary retry after receiving a non-timeout failure from the mpm_get_completion_context() function.
Currently, the only possible reasons why mpm_get_completion_context() could fail are real errors such as being unable to WaitForSingleObject(), allocate memory or create an event. Retrying under such circumstances doesn't make sense, and could be as well considered harmful.
mpm_winnt: Factor out a helper function to parse the type of an accept filter and use an appropriate enum for it.
This makes the code in winnt_accept() a bit easier to follow. As a minor side effect, it also fixes a small bug where the "unrecognized AcceptFilter '%s'" log entry would always contain "none" instead of the actually unrecognized kind of the accept filter.