[MEDIUM] session: better fix for connection to servers with closed input

[MEDIUM] session: better fix for connection to servers with closed input

The following patch fixed an issue but brought another one :
  296897 [MEDIUM] connect to servers even when the input has already been closed

The new issue is that when a connection is inspected and aborted using
TCP inspect rules, now it is sent to the server before being closed. So
that test is not satisfying. A probably better way is not to prevent a
connection from establishing if only BF_SHUTW_NOW is set but BF_SHUTW
is not. That way, the BF_SHUTW flag is not set if the request has any
data pending, which still fixes the stats issue, but does not let any
empty connection pass through.

Also, as a safety measure, we extend buffer_abort() to automatically
disable the BF_AUTO_CONNECT flag. While it appears to always be OK,
it is by pure luck, so better safe than sorry.