fix savecore so that it works on the alpha after the size change

of dumpmag from an int to a u_long in rev 1.41 -- without this
change, savecore will always fail like this:

#savecore -v /var/crash
dumplo = 874356736 (1707728 * 512)
savecore: magic number mismatch (8fca0101 != 8fca0101)
savecore: no core dump

sbin/savecore/savecore.c

@@ -328,7 +328,7 @@ check_kmem()
 void
 clear_dump()
 {
-	int newdumpmag;
+	u_long newdumpmag;
 
 	newdumpmag = 0;
 	DumpWrite(dumpfd, &newdumpmag, sizeof(newdumpmag),
@@ -343,7 +343,7 @@ clear_dump()
 int
 dump_exists()
 {
-	int newdumpmag;
+	u_long newdumpmag;
 
 	DumpRead(dumpfd, &newdumpmag, sizeof(newdumpmag),
 	    (off_t)(dumplo + ok(dump_nl[X_DUMPMAG].n_value)), L_SET);