Make URxvt scrolling more controllable

This should enable reading upper parts of window even while the output
is still being produced (tail -f).

IOW, the auto-scroll is disabled after scrolling back manually using
S-PageUp or S-PageDown, and re-enabled on another keypress such as
typing when a prompt is expected at the bottom.

dotfiles.config/Xlib/global.Xdefaults

 URxvt.scrollBar_floating:   true
 URxvt.scrollstyle:          plain
 URxvt.scrollWithBuffer:     true
+URxvt.scrollTtyOutput:      false
+URxvt.scrollTtyKeypress:    true
 URxvt.saveLines:            32767
 
 !## cursor & pointer