vt(4): When creating a window buffer, fill it entirely

... not just the visible part.

This fixes a bug where, when switching from eg. vt_vga to vt_fb (ie. the
resolution goes up), the originally hidden, uninitialized area of the
buffer is displayed on the screen. This leads to a missing text cursor
when it's over an unitialized area.

This was also visible when selecting text: the uninitialized area was
not highlighted.

Internally, this area was zeroed: characters were all 0x00000000,
meaning the foreground and background color was black. Now, everything
is filled with a space with a gray foreground color, like the visible
area.

While here, remove the check for the mute flag and always use
TERMINAL_NORM_ATTR as the character attribute (ie. gray foreground,
black background).

MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270667 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f