Renaming non-X x_* procedures in xdisp.c (and elsewhere)

[Alex]

It would be helpful to determine at a glance whether or not a procedure
is generic or depends on X support. I'm guessing that this naming scheme
is a historical artifact from when graphical support meant X support,
but I believe that it is more confusing than it is worth at this point.

For example, x_write_glyphs could become disp_write_glyphs.