Move preopen_dir handlers from wasmtime to wasi-common

src/sys/unix/mod.rs

@@ -2,6 +2,14 @@ pub mod fdentry;
 mod host_impl;
 pub mod hostcalls;
 
-pub fn dev_null() -> std::fs::File {
-    std::fs::File::open("/dev/null").expect("failed to open /dev/null")
+use std::fs::File;
+use std::io;
+use std::path::Path;
+
+pub fn dev_null() -> File {
+    File::open("/dev/null").expect("failed to open /dev/null")
+}
+
+pub fn preopen_dir<P: AsRef<Path>>(path: P) -> io::Result<File> {
+    File::open(path)
 }