MFC r302500,r319339,r319543,r319544,r319551,r321138:

r302500 (by cem):

dd(1): Enable access to SIZE_T_MAX character devices

On machines where SIZE_T_MAX exceeds OFF_MAX (signed 64-bit), permit seeking
character devices to negative off_t values. This enables dd(1) to interact
with kernel KVA in /dev/kmem on amd64, for example.

r319339 (by asomers):

Fix integer overflow detection in dd

dd(1) tried to detect whether the seek offset would overflow, but it failed
to account for the case where the provided argument was negative and the
file was a regular file (negative seeks are allowed for character devices).
I fixed it, and added a regression test.

CID: 1368659

r319543:

Stylistic tweaks

Move opening braces of functions from the last column to column 0.

MFC with: r319339

r319544:

Mark :seek_overflow as an expected failure

MFC with: r319339
PR: 219757

r319551 (by asomers):

Fix bin/dd/dd2_tests:seek_overflow on UFS and TMPFS

Split the postive and negative parts into separate test cases. The positive
test case can only run on ZFS, because only ZFS supports files that large.

PR: 219757

r321138:

Remove unnecessary make logic added in r319339

This makes the change cleaner and easier to backport to ^/stable/10.

MFC r289487:

Integrate contrib/netbsd-tests/bin/dd into the FreeBSD test suite as
bin/dd/tests

Ensure fdescfs is mounted on /dev/fd/ for the length testcase as it's used
in validating the characters read from /dev/zero

Sponsored by: EMC / Isilon Storage Division