Change parameters of moveto() and lineto() to signed

The corresponding members of struct vec are signed, as are most of the
arguments passed to the functions, so it's silly that the parameters
themselves are unsigned.  This takes the number of warnings under
clang -Weverything down from 126 to 30.

This doesn't cause any change to the output TTX files.

bedstead.c

@@ -3702,7 +3702,7 @@ clearpath(void)
 }
 
 static void
-moveto(unsigned x, unsigned y)
+moveto(int x, int y)
 {
 	struct point *p = &points[nextpoint++];
 
@@ -3711,7 +3711,7 @@ moveto(unsigned x, unsigned y)
 }
 
 static void
-lineto(unsigned x, unsigned y)
+lineto(int x, int y)
 {
 	struct point *p = &points[nextpoint++];