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/libsh/error.rs

@@ -201,6 +201,7 @@ impl ShErr {
   pub fn is_flow_control(&self) -> bool {
     self.kind.is_flow_control()
   }
+	/// Promotes a shell error from a simple error to an error that blames a span
   pub fn promote(mut self, span: Span) -> Self {
     if self.notes.is_empty() {
       return self;
@@ -208,7 +209,9 @@ impl ShErr {
     let first = self.notes[0].clone();
     if self.notes.len() > 1 {
       self.notes = self.notes[1..].to_vec();
-    }
+    } else {
+			self.notes = vec![];
+		}
 
     self.labeled(span, first)
   }