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/sys_random/mod.rs

@@ -14,5 +14,6 @@ pub unsafe extern "C" fn getrandom(buf: *mut c_void, buflen: size_t, flags: c_ui
         slice::from_raw_parts_mut(buf as *mut u8, buflen as usize),
         flags,
     )
+    .map(|read| read as ssize_t)
     .or_minus_one_errno()
 }