New stat structure.

* VFS and installed MFSes must be in sync before and after this change *

Use struct stat from NetBSD. It requires adding new STAT, FSTAT and LSTAT
syscalls. Libc modification is both backward and forward compatible.

Also new struct stat uses modern field sizes to avoid ABI
incompatibility, when we update uid_t, gid_t and company.
Exceptions are ino_t and off_t in old libc (though paddings added).

lib/libvtreefs/stadir.c

@@ -5,6 +5,7 @@
 #include <time.h>
 #include <sys/statfs.h>
 #include <sys/statvfs.h>
+#include <string.h>
 
 /*===========================================================================*
  *				fs_stat					     *
@@ -22,6 +23,8 @@ PUBLIC int fs_stat(void)
 	if ((node = find_inode(fs_m_in.REQ_INODE_NR)) == NULL)
 		return EINVAL;
 
+	memset(&statbuf, 0, sizeof(struct stat));
+
 	/* Fill in the basic info. */
 	statbuf.st_dev = fs_dev;
 	statbuf.st_ino = get_inode_number(node);