Issues around discovery failure

[honglh]

This PR address the often occurred discovery failures

1. the ssdp_search sets the MX value to 10, allowing the device to have upto 10 seconds to respond. Yet, the RESPONSE_TIMEOUT is set to 5 seconds. The wait loop may exit too early.

In the PR, set MX to 3 which should be enough for today's network (home broadband), and reasonably within the wait period (RESPONSE_TIMEOUT)

2. socket descriptors and wait time should be reset on every select() call

3. change location header search to case-insensitive search

4. remove the check for optional (space) after "Application-URL" in HTTP header (otherwise the URL gets dropped)