Rename Emacs-specific INFINITY constants

Rename Emacs-specific INFINITY constants

Although these constants were not erroneous, as they were
used only in modules that did not include <math.h>, it's
less confusing to names that cannot be confused with
the now-standard INFINITY macro.
* src/dispextern.h (SCROLL_INFINITY): New constant.
* src/dispnew.c, src/scroll.c (INFINITY):
Remove.  All uses replaced with SCROLL_INFINITY.
* src/process.c (wait_reading_process_output):
Rename private constant.