Decode paths associated with file descriptors returned by syscalls

Decode paths associated with file descriptors returned by syscalls

* defs.h (RVAL_FD): New macro.
(RVAL_MASK, RVAL_STR, RVAL_NONE): Update.
* desc.c (sys_dup, sys_delete_module): New functions.
(do_dup2, decode_open, sys_creat): Change return value to RVAL_FD.
* linux/dummy.h (sys_delete_module, sys_dup): Remove.
* linux/syscall.h (sys_delete_module, sys_dup): New prototypes.
* syscall.c (trace_syscall_exiting): Handle RVAL_FD.

Signed-off-by: Zubin Mithra <zubin.mithra@gmail.com>