Fix a panic which occurs in a VIMAGE-enabled kernel after r270158, and
separate socket_hhook_register() part and put it into VNET_SYS{,UN}INIT()
handler.

Discussed with:	marcel
Check the validity of struct sigaction sa_flags value, reject unknown
flags.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Ensure that sigaction flags for signal, which disposition is reset to
ignored or default, are not leaking.  Apparently, there exists code
which relies on SA_SIGINFO not reported for SIG_DFL or SIG_IGN.

In kern_sigaction, ignore flags when resetting.  Encapsulate the flag
and disposition testing into helper sigact_flag_test().

On exec, and when delivering signal with SA_RESETHAND flag set,
signals are reset automatically.  Use new helper sigdflt(), which
removes duplicated code and corrects all flag bits for the signal.

For proc0, set sigintr bit for all ignored signals.  Ignored signals
are consumed in tdsendsignal() and not delivered to the victim thread
at all.

Reported and tested by:	royger
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
vt(4): Add new vd_bitblt_text_t callback, and implement it for vt_vga

Compared to the deprecated vd_bitbltchr_t callback, vd_bitblt_text_t
receives:
    o  the whole text buffer
    o  the dirty area
    o  the mouse cursor (map, position, colors)

This allows the backend to perform optimization on how to draw things.
The goal is to remove vd_bitbltchr_t and vd_putchar_t, once all driver
are converted (only vt_vga is included in this commit).

In vt_vga, this allows to draw the text and the cursor in one pass,
without ever reading from video memory (because it has all the context).
The main benefit is the speed improvement: no more slideshow during
boot!

Other bugs fixed in vt_vga are:
    o  left-most characters are drawn properly (the left-most pixels were
       missing with bold characters and some wide letters such as 'm')
    o  no more black square around the cursor
    o  no cursor flickering when the text is scrolling

There are still many problems to fix: the known issues are marked with
"FIXME" inside the code.

MFC after:	1 week
vt_vga: Clip the draw area to never draw offscreen

This fixes a bug when two windows use different fonts, but a longer-term
solution is required. The dirty area should be stored as pixels, not
character cells, because such coordinates don't have the same meaning in
all windows, when using different fonts.

MFC after:	1 week
Fix a recursive lock acquisition in vi_reset_dev().

Reviewed by:	grehan
We should never enter the PROBE_SETAN phase if we're not ATAPI, since
that's ATAPI specific. Instead, skip to PROBE_SET_MULTI instead for
non ATAPI protocols. The prior code incorrectly terminated the probe
with a break, rather than arranging for probedone to get called. This
caused panics or worse on some systems.
vt(4): Mark new mouse position as dirty only when it's actually displayed

MFC after:	1 week
vt(4): Store cursor bitmap & colors in struct vt_device

This removes the need to specify them to each call to vd_bitblt_text_t
and, therefore, simplifies the API.

MFC after:	1 week