MEDIUM: http: forward headers again while waiting for connection to complete

MEDIUM: http: forward headers again while waiting for connection to complete

Thanks to the last updates on the message pointers, it is now safe again to
enable forwarding of the request headers while waiting for the connection to
complete because we know how to safely rewind this part.

So this patch slightly modifies what was done in commit 80a92c0 ("BUG/MEDIUM:
http: don't start to forward request data before the connect") to let up to
msg->sov bytes be forwarded when waiting for the connection. The resulting
effect is that a POST request may now be sent with the connect's ACK, which
still saves a packet and may even be useful later when TFO is supported.