Update with-exceptions for new (get-opt) API

The function returns a single value rather than a list of a single
value now.

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Use call-with-blocked-async to avoid races with errno

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>
Test errno behaviour

Using ‘tc-drain’ on a regular file should fail. That should cause
‘errno’ to be non-zero. The non-exception interface needs to explore
the value itself.  The with-exceptions module throws 'system-error
with appropriately set arguments

Signed-off-by: Frank Terbeck <ft@bewatermyfriend.org>