filedesc: plug unnecessary fdp NULL checks in fdescfreee and fdcopy

Anything reaching these functions has fd table.
Integrate usr.sbin/useradd/t_useradd.sh from NetBSD into FreeBSD as pw_test

- Suffix useradd/userdel commands with pw
- Remove the atf_expect_fail for bin/39546

Phabric: D921
Reviewed by: brd
Sponsored by: EMC / Isilon Storage Division
filedesc: plus sys/kdb.h include which crept in with r274007
Integrate pjdfstest test suite execution into kyua

pjdfstest execution is opt-in and must be done as root due to some of the
assumptions made by the test suite and lack of error checking in the non-root
case

A description of how to execute pjdfstest with kyua is provided in
share/pjdfstest/README

Phabric: D824 (an earlier prototype patch)
MFC after: 1 month
Relnotes: yes
Sponsored by: EMC / Isilon Storage Division
Provide an on-stack temporary buffer for small ioctl requests.
Reduce boot verbosity.

MFC:		3 days
Remove explicit KYUAFILE=yes from tests/sys/Makefile

This causes the directory traversing Kyuafile to be installed, which in turn
causes tests/sys/pjdfstest to always be run from /usr/tests/sys

Let KYUAFILE default to auto, so the file generated by suite.test.mk gets
installed instead

MFC after: 1 week
Require root when running the pw_delete and pw_modify test programs (pw(8)
requires it)
When other end of the pipe closed during the write, but some bytes
were written, return short write instead of EPIPE.

Update comment.

Discussed with:	bde (long time ago)
MFC after:	2 weeks
drm: Take vt(4) default mode from loader tunables

By default, vt(4) gets the "preferred mode" from DRM, when using a DRM
video driver as its backend. The preferred mode is usually the native
screen resolution.

Now, if this mode isn't appropriate, a user can use loader tunables to
select a mode. The tunables are read in the following order:
    1. kern.vt.fb.modes.$connector_name
    2. kern.vt.fb.default_mode

For example, to set a 1024x768 mode, no matter the connector:
    kern.vt.fb.default_mode="1024x768"

To set a 800x600 mode only on the laptop builtin screen:
    kern.vt.fb.modes.LVDS-1="800x600"

MFC after:	1 week