minimal implementation of ftruncate.

Reference: /n/sources/patch/applied/ape-ftruncate
Date: Sat Aug 23 18:58:10 CES 2008
Signed-off-by: bichued@gmail.com

--- /sys/include/ape/unistd.h	Sat Aug 23 18:57:23 2008
+++ /sys/include/ape/unistd.h	Sat Aug 23 18:57:17 2008
@@ -140,6 +140,7 @@
 extern ssize_t read(int, void *, size_t);
 extern ssize_t write(int, const void *, size_t);
 #ifdef __TYPES_H
+extern int ftruncate(int, off_t);
 extern off_t lseek(int, off_t, int);
 #endif
 
--- /sys/src/ape/lib/ap/plan9/ftruncate.c	Sat Aug 23 18:57:35 2008
+++ /sys/src/ape/lib/ap/plan9/ftruncate.c	Sat Aug 23 18:57:29 2008
@@ -1,10 +1,23 @@
+#include "lib.h"
 #include <sys/types.h>
 #include <unistd.h>
 #include <errno.h>
+#include "dir.h"
 
 int
 ftruncate(int fd, off_t length)
 {
-	errno = EINVAL;
-	return -1;
+	Dir d;
+
+	if(length < 0){
+		errno = EINVAL;
+		return -1;
+	}
+	_nulldir(&d);
+	d.length = length;
+	if(_dirfwstat(fd, &d) < 0){
+		_syserrno();
+		return -1;
+	}
+	return 0;
 }