Secret memory with memfd_secret (#321)

Implements:
- An additional allocator to use memfd_secret(2) and guard pages using mmap(2), implemented in quininer/memsec#16
- An allocator that abstracts away underlying allocators, and uses specified allocator set by rosenpass_secret_memory::policy functions (or a function that sets rosenpass_secret_memory::alloc::ALLOC_INIT
- Updates to tests- integration, fuzz, bench: some tests use procspawn to spawn multiple processes with different allocator policies

wireguard-broker/src/brokers/netlink.rs

@@ -77,7 +77,7 @@ impl WireGuardBroker for NetlinkWireGuardBroker {
     fn set_psk(&mut self, config: SerializedBrokerConfig) -> Result<(), Self::Error> {
         let config: NetworkBrokerConfig = config
             .try_into()
-            .map_err(|e| SetPskError::NoSuchInterface)?;
+            .map_err(|_e| SetPskError::NoSuchInterface)?;
         // Ensure that the peer exists by querying the device configuration
         // TODO: Use InvalidInterfaceError