fbd: Fix a bug where vt_fb_attach() success would be considered a failure

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
vt(4): Add vtbuf_dirty*_locked() to lock vtbuf once, not twice

In several functions, vtbuf_putchar() in particular, the lock on vtbuf
is acquired twice:
    1. once by the said functions;
    2. once in vtbuf_dirty().

Now, vtbuf_dirty_locked() and vtbuf_dirty_cell_locked() allow to
acquire that lock only once.

This improves the input speed of vt(4). To measure the gain, a
50,000-lines file was displayed on the console using cat(1). The time
taken by cat(1) is reported below:
    o  On amd64, with vt_vga:
        - before: 1.0"
	- after:  0.5"
    o  On sparc64, with creator_vt:
        - before: 13.6"
	- after:  10.5"

MFC after:	1 week