MEDIUM: stream-int: replace occurrences of si->appctx with si_appctx()

MEDIUM: stream-int: replace occurrences of si->appctx with si_appctx()

We're about to remove si->appctx, so first let's replace all occurrences
of its usage with a dynamic extract from si->end. A lot of code was changed
by search-n-replace, but the behaviour was intentionally not altered.

The code surrounding calls to stream_int_register_handler() was slightly
changed since we can only use si->end *after* the registration.