basic/in-addr-util: add IN_ADDR_TO_STRING

Since we don't need the error value, and the buffer is allocated with a fixed
size, the whole logic provided by in_addr_to_string() becomes unnecessary, so
it's enough to wrap inet_ntop() directly.

inet_ntop() can only fail with ENOSPC. But we specify a buffer that is supposed
to be large enough, so this should never fail. A bunch of tests of this are added.
This allows all the wrappers like strna(), strnull(), strempty() to be dropped.

The guard of 'if (DEBUG_LOGGING)' can be dropped from around log_debug(),
because log_debug() implements the check outside of the function call. But
log_link_debug() does not, so it we need it to avoid unnecessary evaluation of
the formatting.

src/basic/in-addr-util.c

@@ -444,7 +444,7 @@ int in_addr_to_string(int family, const union in_addr_union *u, char **ret) {
                 return -ENOMEM;
 
         errno = 0;
-        if (!inet_ntop(family, u, x, l))
+        if (!typesafe_inet_ntop(family, u, x, l))
                 return errno_or_else(EINVAL);
 
         *ret = TAKE_PTR(x);