sys/kern: Add VFS support

- Refactor the libio interface

- Move syscalls into an rtemsbsd location

- Provide a root directory mount point

Update #4475

rtemsbsd/include/vm/vm_pager.h

@@ -0,0 +1,20 @@
+#ifndef RTEMS_VM_PAGER_H
+#define RTEMS_VM_PAGER_H
+
+/*
+ * get/put return values
+ * OK    operation was successful
+ * BAD   specified data was out of the accepted range
+ * FAIL  specified data was in range, but doesn't exist
+ * PEND  operations was initiated but not completed
+ * ERROR error while accessing data that is in range and exists
+ * AGAIN temporary resource shortage prevented operation from happening
+ */
+#define VM_PAGER_OK     0
+#define VM_PAGER_BAD    1
+#define VM_PAGER_FAIL   2
+#define VM_PAGER_PEND   3
+#define VM_PAGER_ERROR  4
+#define VM_PAGER_AGAIN  5
+
+#endif