server: Change the removable fd flag to a cacheable flag to make it possible for fd users to set it dynamically.

server/protocol.def

diff --git a/server/protocol.def b/server/protocol.def
index e41cd18..5f1a8f9 100644
--- a/server/protocol.def
+++ b/server/protocol.def
@@ -1018,7 +1018,7 @@
     obj_handle_t handle;        /* handle to the file */
 @REPLY
     int          type;          /* file type (see below) */
-    int          removable;     /* is file removable? */
+    int          cacheable;     /* can fd be cached in the client? */
     unsigned int access;        /* file access rights */
     unsigned int options;       /* file open options */
 @END