Makes disk head position explicitly something with sub-integral precision.

Also as a drive-by fix, corrects accidental assumption of 10 sectors for all MFMSectorDump descendants.

Components/1770/1770.cpp

@@ -282,7 +282,7 @@ void WD1770::posit_event(int new_event_type) {
 			track_ = 0;
 			goto verify;
 		}
-		get_drive().step(step_direction_ ? 1 : -1);
+		get_drive().step(Storage::Disk::HeadPosition(step_direction_ ? 1 : -1));
 		unsigned int time_to_wait;
 		switch(command_ & 3) {
 			default:

Components/8272/i8272.cpp

@@ -115,7 +115,7 @@ void i8272::run_for(Cycles cycles) {
 					int direction = (drives_[c].target_head_position < drives_[c].head_position) ? -1 : 1;
 					printf("Target %d versus believed %d\n", drives_[c].target_head_position, drives_[c].head_position);
 					select_drive(c);
-					get_drive().step(direction);
+					get_drive().step(Storage::Disk::HeadPosition(direction));
 					if(drives_[c].target_head_position >= 0) drives_[c].head_position += direction;
 
 					// Check for completion.

Components/DiskII/DiskII.cpp

@@ -57,7 +57,7 @@ void DiskII::set_control(Control control, bool on) {
 
 		// Compare to the stepper position to decide whether that pulls in the current cog notch,
 		// or grabs a later one.
-		drives_[active_drive_].step(-direction);
+		drives_[active_drive_].step(Storage::Disk::HeadPosition(-direction, 4));
 		stepper_position_ = (stepper_position_ - direction + 8) & 7;
 	}
 }