implemented '<>' redirects, and the 'seek' builtin

'seek' is a wrapper around the lseek() syscall

added noclobber to core shopts and implemented '>|' redirection syntax

properly implemented fd close syntax

fixed saved fds being leaked into exec'd programs

src/prelude.rs

@@ -19,7 +19,7 @@ pub use std::os::unix::io::{AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd,
 pub use bitflags::bitflags;
 pub use nix::{
   errno::Errno,
-  fcntl::{OFlag, open},
+  fcntl::{FcntlArg, OFlag, fcntl, open},
   libc::{self, STDERR_FILENO, STDIN_FILENO, STDOUT_FILENO},
   sys::{
     signal::{self, SigHandler, SigSet, SigmaskHow, Signal, kill, killpg, pthread_sigmask, signal},