properly store IPv6 addresses in struct tun_t

All addresses in struct tun_t were stored as an in_addr.
But IPv6 addresses need an in6_addr, so switch tun_t addresses
to the in64_addr wrapper struct.

This is an ABI break, as documented in TODO-RELEASE.

Fixes an out of bounds memcpy() identified by Coverity.

Change-Id: Idd2431ad25d7fa182e52e2bd5231ceb04d427c34
Related: CID#174278
diff --git a/TODO-RELEASE b/TODO-RELEASE
index d0852fc..bbad169 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -7,3 +7,4 @@
 # If any interfaces have been added since the last public release: c:r:a + 1.
 # If any interfaces have been removed or changed since the last public release: c:r:0.
 #library	what		description / commit summary line
+libmisc		tun_t		tun_t structure has grown due to switch from in_addr to in46_addr (g#11870)