Check return value of dst_key_getbool()

Check return value of dst_key_getbool()

Fix Coverity CHECKED_RETURN reports for dst_key_getbool().  In most
cases we do not really care about its return value, but it is prudent
to check it.

In one case, where a dst_key_getbool() error should be treated
identically as success, cast the return value to void and add a relevant
comment.