Finish downcasing mouse_init1, mouse_off,

Finish downcasing mouse_init1, mouse_off,
mouse_check_moved, mouse_pressed, mouse_released, mouse_on, mouse_init.

(visible_bell): Now static.  Make the asm volatile.
Don't expect register %gs to survive calls to other functions
(just to be safe, %gs will survive at present).

(glyph_to_pixel_coords, pixel_to_glyph_coords) new
functions imitating those in xterm.c.

(dos_rawgetc): Key numbers are biased by 0xff00.