Handle EOF on one side of a UnixSocketRelay. (#323)

- Current implementation shuts down everything as soon as EOF is
detected on DispatchSourceRead for the relay. This is problematic for,
say an HTTP request where the client makes a request, and calls
`shutdown(fd, SHUT_WR)` to close the send side, but it expects to be
able to keep calling `recv()` to get the response.
- Changed cancel handlers so that the one that sees that both sources
have been cancelled then closes both the UDS and vsock fds.
- Updated vminitd VsockProxy to also do `shutdown(fd, SHUT_WR)` for read
hangup or EOF, and only close the underlying fds and unwire pollers on
full hangup, broken pipe, error, or when both sides half-close.

Sources/ContainerizationOS/Linux/Epoll.swift

@@ -169,7 +169,11 @@ public final class Epoll: Sendable {
 
 extension Epoll.Mask {
     public var isHangup: Bool {
-        (self & (EPOLLHUP | EPOLLERR | EPOLLRDHUP)) != 0
+        (self & (EPOLLHUP | EPOLLERR)) != 0
+    }
+
+    public var isRhangup: Bool {
+        (self & EPOLLRDHUP) != 0
     }
 
     public var readyToRead: Bool {