MEDIUM: stream-int: split the shutr/shutw functions between applet and conn

MEDIUM: stream-int: split the shutr/shutw functions between applet and conn

These functions induce a lot of ifs everywhere because they consider two
different cases, one which is where the connection exists and has a file
descriptor, and the other one which is the default case where at most an
applet has to be notified.

Let's have them in si_ops and automatically decide which one to use.

The connection shutdown sequence has been slightly simplified, and we
now clear the flags at the end.

Also we remove SHUTR_NOW after a shutw with nolinger, as it's cleaner
not to keep it.