Core: disabled SO_REUSEPORT when testing config (ticket #1300). When closing a socket with SO_REUSEPORT, Linux drops all connections waiting in this socket's listen queue. Previously, it was believed to only result in connection resets when reconfiguring nginx to use smaller number of worker processes. It also results in connection resets during configuration testing though. Workaround is to avoid using SO_REUSEPORT when testing configuration. It should prevent listening sockets from being created if a conflicting socket already exists, while still preserving detection of other possible errors. It should also cover UDP sockets. The only downside of this approach seems to be that a configuration testing won't be able to properly report the case when nginx was compiled with SO_REUSEPORT, but the kernel is not able to set it. Such errors will be reported on a real start instead.