make format_host non-reentrant by default

There are only three users which require it to be reentrant, the rest is
fine without. Instead, provide a reentrant format_host_r() for users
which need it.

Signed-off-by: Phil Sutter <phil@nwl.cc>
diff --git a/ip/ip6tunnel.c b/ip/ip6tunnel.c
index 2e9d3ed..d588645 100644
--- a/ip/ip6tunnel.c
+++ b/ip/ip6tunnel.c
@@ -77,7 +77,7 @@
 	printf("%s: %s/ipv6 remote %s local %s",
 	       p->name,
 	       tnl_strproto(p->proto),
-	       format_host(AF_INET6, 16, &p->raddr, s1, sizeof(s1)),
+	       format_host_r(AF_INET6, 16, &p->raddr, s1, sizeof(s1)),
 	       rt_addr_n2a(AF_INET6, 16, &p->laddr, s2, sizeof(s2)));
 	if (p->link) {
 		const char *n = ll_index_to_name(p->link);