unix: add if_indextoname

if_indextoname(...) is a function defined in <net/if.h>

It is used to convert an IPv6 scope_id into an interface
identifier string such as %eth0 or %lo

There is an identical function in Windows, but
that has not been tested.

Adds documentation and an ASSERT to the relevant test case.

PR-URL: libuv#1445

docs/src/misc.rst

@@ -271,6 +271,12 @@ API
     and :man:`inet_pton(3)`. On success they return 0. In case of error
     the target `dst` pointer is unmodified.
 
+.. c:function:: char* uv_if_indextoname(unsigned int ifindex, char* ifname)
+
+    Cross-platform IPv6-capable implementation of :man:`if_indextoname(3)`.
+    Writes the result to `ifname`, which must be at least `IFNAMSIZ` long.
+    On success returns the generated string.  In case of error returns `NULL`.
+
 .. c:function:: int uv_exepath(char* buffer, size_t* size)
 
     Gets the executable path.

include/uv-unix.h

@@ -32,6 +32,7 @@
 #include <netinet/tcp.h>
 #include <arpa/inet.h>
 #include <netdb.h>
+#include <net/if.h>
 
 #include <termios.h>
 #include <pwd.h>

include/uv.h

@@ -1392,6 +1392,8 @@ UV_EXTERN int uv_ip6_name(const struct sockaddr_in6* src, char* dst, size_t size
 UV_EXTERN int uv_inet_ntop(int af, const void* src, char* dst, size_t size);
 UV_EXTERN int uv_inet_pton(int af, const char* src, void* dst);
 
+UV_EXTERN char* uv_if_indextoname(unsigned int ifindex, char* ifname);
+
 UV_EXTERN int uv_exepath(char* buffer, size_t* size);
 
 UV_EXTERN int uv_cwd(char* buffer, size_t* size);

src/inet.c

@@ -49,6 +49,9 @@ int uv_inet_ntop(int af, const void* src, char* dst, size_t size) {
   /* NOTREACHED */
 }
 
+char* uv_if_indextoname(unsigned int ifindex, char* ifname) {
+    return if_indextoname(ifindex, ifname);
+}
 
 static int inet_ntop4(const unsigned char *src, char *dst, size_t size) {
   static const char fmt[] = "%u.%u.%u.%u";

test/test-ip6-addr.c

@@ -67,6 +67,8 @@ TEST_IMPL(ip6_addr_link_local) {
     iface_index = address->address.address6.sin6_scope_id;
     device_name = address->name;
 
+    ASSERT(0 == strcmp(device_name, uv_if_indextoname(iface_index, scoped_addr)));
+
 #ifdef _WIN32
     snprintf(scoped_addr,
              sizeof(scoped_addr),