upstream check: fix the check status "down" for a single server.

If a single server is marked down by upstream check module, it will
return 502 directly before.

This fix can use the backup server eventually for sure in the above
case.
Fixed segmentation fault in ngx_show_dso_directives() and ngx_is_dynamic_module().if we don't set dso block in nginx config, ctx->module is NULL, and then exec nginx -v, -m or -l will casuse nginx coredump.
Merge pull request #149 from monadbobo/dso_fix

Fixed segmentation fault
Merge pull request #148 from yaoweibin/check_single

upstream check: fixed the check status "down" for a single server.
Merge branch 'develop' into request_no_buffering
added socket nodelay for non buffered request when sending data to upstream
recover the read event