Support specififying non-default batch size

Provide new interfaces to allow calling `sendmmsg` and `recvmmsg` with a
non-default batch size, specified by API clients as a generic constant during
compile time. This is presently 1024 on Linux and FreeBSD. The default batch
size is increased to 128 on these platforms.

Also update to minimally support FreeBSD. Not all Linux features are supported
yet.

Author: Scott Hutton

src/lib.rs

@@ -16,8 +16,17 @@ mod imp;
 pub use imp::{sync, UdpSocket};
 pub mod framed;
 
-/// Number of UDP packets to send/receive at a time
-pub const BATCH_SIZE: usize = imp::BATCH_SIZE;
+/// Maximum number of UDP packets that can be sent by the `sendmmsg`/`recvmmsg`
+/// wrappers.  Note that, for supported platforms, the OS caps the batch size at
+/// this value, but will not return an error, so this is just a suggested
+/// maximum.
+///
+/// Presently, this is 1024 on Linux an FreeBS, and 1 on platforms that don't
+/// support `sendmmsg`/`recvmmsg`
+pub const BATCH_SIZE_CAP: usize = imp::BATCH_SIZE_CAP;
+
+/// Default number of UDP packets to send/receive at a time.
+pub const DEFAULT_BATCH_SIZE: usize = imp::DEFAULT_BATCH_SIZE;
 
 /// The capabilities a UDP socket suppports on a certain platform
 #[derive(Debug)]