Add STDIN file number

src/io/mod.rs

@@ -34,6 +34,8 @@ pub struct UnmountOptions {
 }
 
 impl RawFd {
+    /// Default input descriptor of a process
+    pub const STDIN: Self = Self(0);
     /// Default output descriptor of a process
     pub const STDOUT: Self = Self(1);
     /// Error output descriptor of a process