BUG/MEDIUM: applet: Don't call .release callback function twice

BUG/MEDIUM: applet: Don't call .release callback function twice

Since the CS/SI refactoring, the .release callback function may be called
twice. The first call when a shutdown for read or for write is performed.
The second one when the applet is detached from its conn-stream. The second
call must be guarded, just like the first one, to only be performed is the
stream-interface is not the in disconnected (SI_ST_DIS) or closed
(SI_ST_CLO) state.

To simplify the fix, we now always rely on si_applet_release() function.

It is 2.6-specific, no backport is needed.