interface: fs: replace ifc definition with xpcg

interface/fs.xpc

@@ -0,0 +1,9 @@
+interface fs 6400;
+
+func open[0](path: string, flags: int) -> (err: int);
+func close[1]() -> (err: int);
+
+func read[2](count: size) -> (err: int, nr_read: size, data: buffer);
+func write[3](data: buffer) -> (err: int, nr_written: size);
+
+func map[4](prot: int, flags: int) -> (vmo: handle);