sd-icmp6-nd: rename files to sd-ndisc

The actual code rename will follow. The reason for the change of name is to make it
simpler and more uniform with how we name other libraries (we don't include the
underlying protocol). The new name also matches the naming in the kernel (which
is particularly relevent here as we expect to let the kernel do some parts of
the protocol and we do others).

src/libsystemd-network/network-internal.c

@@ -32,7 +32,7 @@
 #include "conf-parser.h"
 #include "condition.h"
 #include "network-internal.h"
-#include "sd-icmp6-nd.h"
+#include "sd-ndisc.h"
 
 const char *net_get_name(struct udev_device *device) {
         const char *name, *field;