Show "screen of death" on VGA console upon program crash (#650)

* [metal] Refactoring: separate out sys_writev_vga() and _vga_init() modules

* [metal] Read VGA info from BDA before long mode entry, not after

If using a pre-existing VGA text console, the VGA initialization
code now retrieves the cursor position & character height from
the BIOS data area while still in real mode — rather than
reading from the BIOS data area only after entering long mode.

(This should help make the code more correct, if Cosmopolitan
were to support UEFI graphics output in the future.  If the
program were booted via UEFI, then the long mode IsMetal()
code would still be activated, but the BIOS data area might
not have been initialized in that case.)

This change also means that there are now a few more fields
in the `struct mman`.

* [metal] VGA console can now show "screen of death" upon a crash

There is now a new function _klog_vga(), which can be called
by kprintf() to output system messages — e.g. information about
CPU exceptions — on the VGA screen.

* [metal] CPU exception handler now dumps cr2 value
* [metal] Add demo of program crash reporting w/ bare metal VGA TTY
* [metal] Reduce size of "screen of death" code

libc/runtime/mman.internal.h

@@ -29,6 +29,12 @@ struct mman {
   uint64_t pc_video_framebuffer;         /* 0x1d30 — physical address of
                                                      video frame buffer */
   uint64_t pc_video_framebuffer_size;    /* 0x1d38 */
+  struct {                               /* 0x1d40 — starting cursor pos. */
+    unsigned short y, x;
+  } pc_video_curs_info;
+  unsigned short pc_video_char_height;   /* 0x1d44 — character height (useful
+                                                     for setting cursor shape
+                                                     in text mode) */
 };
 
 COSMOPOLITAN_C_END_