Update thread and syscall to use local fd counter instead global one, preventing overflow

src/threads/thread.h

@@ -32,6 +32,9 @@ typedef int tid_t;
 #define NICE_DEFAULT 0 /* Default niceness. */
 #define NICE_MAX 20    /* Highest niceness. */
 
+/* File Descriptors. */
+#define MINIMUM_USER_FD 2 /* Minimum file descriptor for user programs. */
+
 /* A process result, synchronised between parent and child. */
 struct process_result
 {
@@ -137,7 +140,9 @@ struct thread
 #ifdef USERPROG
     /* Owned by userprog/process.c. */
     uint32_t *pagedir;                  /* Page directory. */
-    struct hash open_files;             /* Hash Table of FD -> Struct File */
+    unsigned int fd_counter;            /* File descriptor counter for thread's
+                                           open files. */
+    struct hash open_files;             /* Hash Table of FD -> Struct File. */
 #endif
 
     /* Owned by thread.c. */