Use portable_atomic for the interface presence bind counter

The bind counter in the interface presence tracker was a std AtomicU64,
which does not exist on the 32-bit MIPS targets the OpenWrt packages
cover, so it would stop the crate building there. portable_atomic uses
the std type wherever the target has 64-bit atomics, so nothing changes
on other targets. This is the one std 64-bit atomic outside tests that
exists only on this branch; the rest are converted by the maint change
that adds the CI check for them.

src/transport/presence.rs

@@ -29,7 +29,8 @@
 //! rather than steering it. See
 //! the ethernet transport's `interface_present` for why.
 
-use std::sync::atomic::{AtomicU8, AtomicU32, AtomicU64, Ordering};
+use portable_atomic::AtomicU64;
+use std::sync::atomic::{AtomicU8, AtomicU32, Ordering};
 use std::sync::{PoisonError, RwLock, RwLockReadGuard, RwLockWriteGuard};
 use std::time::{Duration, Instant};