vt_fb_attach() currently always returns 0, but it could return a code
defined in errno.h. However, it doesn't return a CN_* code. So checking
its return value against CN_DEAD (which is 0) is incorrect, and in this
case, a success becomes a failure.
The consequence was unimportant, because the caller (drm_fb_helper.c)
would only log an error message in this case. The console would still
work.
Approved by: nwhitehorn
rather than only drivers attached later on. This involves a small amount of
code duplication with dev/fb/fbd.c, which will fixed later on.
Also improve performance of vt_blank() by making it not read from the
framebuffer unnecessarily.
o Forward termianl framebuffer ioctl to fbd.
o Forward terminal mmap request to fbd.
o Move inclusion of sys/conf.h to vt.h.
Sponsored by: The FreeBSD Foundation