Use unsigned return values in read()-like Pal fns.

Returning a negative number of bytes makes absolutely no sense, besides
the "-1 and errno" pattern, which is now converted to Result<_, Errno>.

src/header/stdlib/mod.rs

@@ -986,7 +986,9 @@ pub unsafe extern "C" fn realpath(pathname: *const c_char, resolved: *mut c_char
 
         let len = out.len();
         // TODO: better error handling
-        let read = Sys::fpath(*file, &mut out[..len - 1]).or_minus_one_errno();
+        let read = Sys::fpath(*file, &mut out[..len - 1])
+            .map(|read| read as ssize_t)
+            .or_minus_one_errno();
         if read < 0 {
             return ptr::null_mut();
         }