vt(4): If the cursor is globally disabled, don't mark its position as dirty

This avoids unnecessary redraw. In particular, during boot, where the
cursor is disabled and its fake position is [0;0], this triggered a
refresh of the whole screen each time vt_flush() is called.

MFC after:	1 week
vt(4): If the cursor didn't move, don't mark its position as dirty

Currently, this has no effect, because the cursor is always redrawn
anyway. But this will be useful after improvements to the vd_bitbltchr_t
callback API.

The vt_device structure members used to store the position of the cursor
as of the last redraw are renamed from vd_mdirty{x,y} to vd_mold{x,y}.
The associated comment is fixed too. Also, their value is now expressed
in pixels, not in character columns/row.

MFC after:	1 week
vt(4): Mark the current cursor position as dirty

Like r270273, this has no effect for now, because the cursor is always
drawn. This is in preparation of future changes to vd_bitbltchr_t API.

MFC after:	1 week
Use __FBSDID() properly.

Suggested by:	pluknet@
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Add guards to ptrdiff_t definition in include/stddef.h

Back in 2011 obrien has added the #define macro in sys/sys/stddef.h to
guard ptrdiff_t. Add similar protection to the identical code in
include/stddef.h.

Submitted by:   Mariusz Zaborski <oshogbo@FreeBSD.org>
MFC after:      1 week
vt(4): Mark cursor position as dirty when we enable/disable it

MFC after:	1 week
Make the iSCSI stack use __FBSDID() properly.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
vt(4): Pause the flush timer while swithing window

This fixes bad looking refresh when switching window: squares instead
of text, flashing screen, and so on. In the worst case, vt_flush() came
at a very inappropriate timing and the screen was not refreshed at all
(leaving squares all over the place).

This doesn't fix the flickering of the screen with vt_vga, because the
sync signal is temporarily stopped and the video memory is cleared.

MFC after:	1 week
Fix includes.

Suggested by:	pluknet@
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Use proper include paths in kernel iSCSI code.

MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Add `-A' flag to pkg-install(8) invocation when installing dependencies.

MFC after:	3 days
X-MFC-to:	stable/10, stable/9
Reported by:	gjb
Discussed with:	jelischer, gjb, bdrewery
* Add new net/sff8436.h containing constants used to access
  QSFP+ data via i2c inteface. These constants has been taken
  from SFF-8436 "QSFP+ 10 Gbs 4X PLUGGABLE TRANSCEIVER" standard
  rev 4.8.
* Add support for printing QSFP+ information from 40G NICs
  such as Chelsio T5.

This commit does not contain ioctl changes necessary for this
functionality work, there will be another commit soon.

Example:
cxl1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        options=ec07bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,.....>
        ether 00:07:43:28:ad:08
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
        media: Ethernet 40Gbase-LR4 <full-duplex>
        status: active
        plugged: QSFP+ 40GBASE-LR4 (MPO Parallel Optic)
        vendor: OEM PN: OP-QSFP-40G-LR4 SN: 20140318001 DATE: 2014-03-18
        module temperature: 64.06 C voltage: 3.26 Volts
        lane 1: RX: 0.47 mW (-3.21 dBm) TX: 2.78 mW (4.46 dBm)
        lane 2: RX: 0.20 mW (-6.94 dBm) TX: 2.80 mW (4.47 dBm)
        lane 3: RX: 0.18 mW (-7.38 dBm) TX: 2.79 mW (4.47 dBm)
        lane 4: RX: 0.90 mW (-0.45 dBm) TX: 2.80 mW (4.48 dBm)

Tested on:	Chelsio T5
Tested on:	Mellanox/Huawei passive/active cables/transceivers.
MFC after:	2 weeks
Sponsored by:	Yandex LLC
vt(4): Constify vt_buf argument of vtbuf_iscursor()

MFC after:	1 week
Change file permissions for some setuid executables so they are "o+r".
The executable itself doesn't contain any privileged information.

An example of where this is useful is when makefs(8) is creating an image
that includes /sbin/shutdown. This can now be done without root privileges.

Reviewed by:	delphij
Discussed with:	delphij, des
CR:		https://reviews.freebsd.org/D662
vt(4): Test if the cursor is shown only once

Later, we just see if the "struct mouse_cursor" pointer is set. This
avoids the need to mess with all the conditions several times; this has
been error prone.

While here, rename the variable "m" to a more meaningful "cursor", like
it's done elsewhere in the code.

MFC after:	1 week
vt(4): Rename the "mouse_cursor" structure to "vt_mouse_cursor"

At the same time, "w" and "h" members are now called "width" and
"height". The goal is to have a more "public" structure, because it will
soon be passed as argument to a new callback, replacing vd_bitbltchr_t.

MFC after:	1 week
vt_vga: When clearing video memory, don't read from it

The goal is to clear the video memory, in case an application drew to
it. So the content shouldn't be loaded in the latches, it can't be
trusted anyway.

This improves a bit the window switch speed.

MFC after:	1 week
Next round of fixes.

MFC after:	3 days
Modify a few output file names as used with vt.

MFC after:	3 days
Rename DT_FEATURE_1 to DT_FEATURE

This provides a minor cleanup in elfdump; there are otherwise no
consumers in the tree.  Old SUN documentation can be found for either
variant, but GNU binutils switched to DT_FEATURE around 2000.

Sponsored by:	The FreeBSD Foundation