Shell: Separate fd rewirings from redirections.

This was unnecessarily confusing. When we build up a chain of commands
connected by pipes, we now store the file descriptors of each end of
these pipes as rewirings in a vector. The rewirings are then put into
effect by calls to dup2().

Shell/Parser.h

@@ -10,7 +10,6 @@ struct Redirection {
         FileWrite,
         FileWriteAppend,
         FileRead,
-        Rewire
     };
     Type type;
     int fd { -1 };
@@ -18,9 +17,15 @@ struct Redirection {
     String path {};
 };
 
+struct Rewiring {
+    int fd { -1 };
+    int rewire_fd { -1 };
+};
+
 struct Subcommand {
     Vector<String> args;
     Vector<Redirection> redirections;
+    Vector<Rewiring> rewirings;
 };
 
 class Parser {