checkpath.c: Rename global flags and put their definitions in constact.

checkpath.c: Rename global flags and put their definitions in constact.

The `f_...' naming has been reserved for flags local to a function for a
long time in my private naming conventions.

To make matters worse, the two flags applied in entirely different
context -- and even had the same numerical value.  This is now made
clear by giving them distinct prefices and defining them in different
places.