sys_dup now uses return file descriptor decoding

The following changes were made :-
* defs.h: RVAL_FD introduced. RVAL_MASK, RVAL_STR, RVAL_NONE
  changed.
* desc.c (sys_dup): New function that handles the dup system call.
* linux/dummy.h: sys_dup macro removed.
* syscall.c (trace_syscall_exiting): Use RVAL_FD as to resolve
  returned file descriptors if necessary.
Corrections made to implement sys_open return fd decoding correctly

The following changes were made :-
* linux/syscall.h (sys_dup): New prototype
* defs.h (RVAL_NONE): Macro value corrected