use umount2 syscall for umount (new targets lack old 1-arg umount)

src/linux/umount.c

@@ -1,8 +1,7 @@
 #include <sys/mount.h>
-#define SYSCALL_STANDALONE
 #include "syscall.h"
 
 int umount(const char *special)
 {
-	return syscall1(__NR_umount, (long)special);
+	return syscall2(__NR_umount2, (long)special, 0);
 }