core: Print a dump of the I/O data only at level 8.

* src/debug.h (TRACE_SUC3): New.
(TRACE_LOGBUFX): New.
* src/posix-io.c: Use TRACE_LOGBUFX instead of TRACE_LOGBUF.
* src/w32-glib-io.c: Ditto.

This will also be changed for w32-io as part of another commit.

• Signed-off-by: Werner Koch <wk@gnupg.org>