Limited support for nested FS->VFS requests during VFS->FS call.

- Changed VFS-FS protocol to only store OK or negative error code in
  m_type field of reply messages.
- Changed VFS to treat nonzero positive replies from FS as requests.
- Added backwards compatibility to VFS and MFS.
No protection of global data structures is provided in VFS, so many
VFS calls cannot be made safely by FS servers during many FS calls.
Use with caution (or, preferably, not at all).

include/minix/vfsif.h

@@ -124,6 +124,10 @@
 
 /* For REQ_GETDENTS */
 #define RES_GDE_POS_CHANGE	 m2_l1
+#define RES_GDE_CUM_IO		 m2_i1
+
+/* For REQ_RDLINK */
+#define RES_RDL_LENGTH		 m6_l1
 
 /* Request numbers */
 #define REQ_GETNODE	(VFS_BASE + 1)	/* Should be removed */
@@ -145,7 +149,7 @@
 #define REQ_UNLINK_O	(VFS_BASE + 17)	/* Replaced with REQ_UNLINK_S */
 #define REQ_RMDIR_O	(VFS_BASE + 18)	/* Replaced with REQ_RMDIR_S */
 #define REQ_UTIME	(VFS_BASE + 19)
-#define REQ_RDLINK_S	(VFS_BASE + 20)
+#define REQ_RDLINK_SO	(VFS_BASE + 20) /* Replaced with REQ_RDLINK_S */
 #define REQ_FSTATFS	(VFS_BASE + 21)
 #define REQ_BREAD_S	(VFS_BASE + 22)
 #define REQ_BWRITE_S	(VFS_BASE + 23)
@@ -165,7 +169,7 @@
 #define REQ_NEW_DRIVER	(VFS_BASE + 37)
 #define REQ_BREAD_O	(VFS_BASE + 38)	/* Replaced with REQ_BREAD_S */
 #define REQ_BWRITE_O	(VFS_BASE + 39)	/* Replaced with REQ_BWRITE_S */
-#define REQ_GETDENTS	(VFS_BASE + 40)
+#define REQ_GETDENTS_O	(VFS_BASE + 40) /* Replaced with REQ_GETDENTS */
 #define REQ_FLUSH	(VFS_BASE + 41)
 #define REQ_READ_S	(VFS_BASE + 42)
 #define REQ_WRITE_S	(VFS_BASE + 43)
@@ -178,13 +182,15 @@
 #define REQ_MOUNTPOINT_S (VFS_BASE + 50)
 #define REQ_READSUPER_S	(VFS_BASE + 51)
 #define REQ_NEWNODE	(VFS_BASE + 52)
+#define REQ_RDLINK_S	(VFS_BASE + 53)
+#define REQ_GETDENTS	(VFS_BASE + 54)
 
-#define NREQS                    53
+#define NREQS                    55
 
 
-#define EENTERMOUNT              301 
-#define ELEAVEMOUNT              302
-#define ESYMLINK                 303
+#define EENTERMOUNT              -301 
+#define ELEAVEMOUNT              -302
+#define ESYMLINK                 -303
 
 /* REQ_L_FLAGS */
 #define PATH_RET_SYMLINK	1	/* Return a symlink object (i.e.