net: reorganize headers

Move public headers to include/net, and leave private headers in net/.
Put the virtio headers in include/net/tap.h, removing the multiple copies
that existed.  Leave include/net/tap.h as the interface for NICs, and
net/tap_int.h as the interface for OS-specific parts of the tap backend.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/net/slirp.c b/net/slirp.c
index f117412..5a11ac5 100644
--- a/net/slirp.c
+++ b/net/slirp.c
@@ -29,7 +29,7 @@
 #include <pwd.h>
 #include <sys/wait.h>
 #endif
-#include "net.h"
+#include "net/net.h"
 #include "clients.h"
 #include "hub.h"
 #include "monitor.h"