dd: Better handle user specified offsets that are too big

Following are the before and after operations for seekable files,
for the various erroneous offsets handled by this patch:

skip beyond end of file
  before: immediately exit(0);
  after : immediately printf("cannot skip to specified offset"); exit(0);

skip > max file size
  before: read whole file and exit(0);
  after : immediately printf("cannot skip: Invalid argument"); exit(1);
seek > max file size
  before: immediately printf("truncate error: EFBIG"); exit(1);
  after : immediately printf("truncate error: EFBIG"); exit(1);

skip > OFF_T_MAX
  before: read whole device/file and exit(0);
  after : immediately printf("cannot skip:"); exit(1);
seek > OFF_T_MAX
  before: immediately printf("truncate error: offset too large"); exit(1);
  after : immediately printf("truncate error: offset too large"); exit(1);

skip > device size
  before: read whole device and exit(0);
  after : immediately printf("cannot skip: Invalid argument"); exit(1);
seek > device size
  before: read whole device and printf("write error: ENOSPC"); exit(1);
  after : immediately printf("cannot seek: Invalid argument"); exit(1);

* NEWS: Summarize this change in behavior.
* src/dd.c (skip): Add error checking for large seek/skip offsets on
seekable files, rather than deferring to using read() to advance offset.
(dd_copy): Print a warning if skip past EOF, as per FIXME comment.
* test/Makefile.am: Add 2 new tests.
* tests/dd/seek-skip-past-file: Add tests for first 3 cases above.
* tests/dd/seek-skip-past-dev: Add root only test for last case above.

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