Merge branch 'sample-http-server'

Some improvements for http-server sample:
- getopt
- persistent port via -p option
- IOCP for win32 via -I
- disable buffering
- enable debug logging via -v/EVENT_DEBUG_LOGGING_ALL
- cleanup (by signal and separate error path on errors)

* sample-http-server:
  s/http-server: graceful cleanup
  s/http-server: enable debug logging if EVENT_DEBUG_LOGGING_ALL env isset
  s/http-server: turn off buffering (otherwise do output on win32)
  s/http-server: add an option to use IOCP
  s/http-server: add options (for persistent port)

Refs: #709