small cleanup of #ifdefs in BindListenPort()

- SO_NOSIGPIPE isn't available on WIN32 so merge the 2 non-WIN32 blocks
- use predefined names from header for IPV6_PROTECTION_LEVEL and
  PROTECTION_LEVEL_UNRESTRICTED
add missing BOOST_FOREACH indentation in ThreadSocketHandler()
Merge pull request #4397

5d59921 add missing BOOST_FOREACH indentation in ThreadSocketHandler() (Philip Kaufmann)
9e9ca2b small cleanup of #ifdefs in BindListenPort() (Philip Kaufmann)