add  TCP SO_KEEPALIVE support for HTTP upstream.

Add TCP SO_KEEPALIVE support for HTTP upstream.

test conf: 
```
......
upstream backend {
    server  www.baidu.com;

    keepalive 16; #optional.
    so_keepalive tcp_keepidle=3 tcp_keepintvl=3 tcp_keepcnt=3;
}
...........
location ^~/index.html {
    proxy_pass http://backend/;
    proxy_http_version 1.1;
    proxy_set_header Connection "";
    proxy_set_header Host www.baidu.com;
}
```
Because not every OS support modifying these three so_keepalive parameters, and this MACRO is not defined in the code of these commit, is just follow the usage of Nginx itself.