nghttpx: Treat '*' in <HOST> parameter of --frontend as wildcard explicitly

It seems that specifyig '*' to node parameter in getaddrinfo() is
treated as specifying NULL, but it is not documented.  So rather than
relying on this feature, we explicitly treat '*' as "wildcard" address
and specify NULL to node parameter in getaddrinfo().

Now '*,3000' is a default value of --frontend option.  Specyfing '*'
binds all addresses including both IPv4 and IPv6.