Add a nslookup_pref global parameter

This parameter allows to change the name resolution and connection
preference for fcgi servers and revproxy URLs. It takes a list of the
form [inet | inet6].

For instance, to perform only IPv4 resolution:
    nslookup_pref = [inet]

To perform both IPv4 and IPv6, but try IPv6 first:
    nslookup_pref = [inet6, inet]

The default value is [inet]. Therefore, the behavior remains the same
with this addition.

This parameter is used by two new functions:
    o  yaws:tcp_connect/{3,4}
    o  yaws:ssl_connect/{3,4}

They are wrappers around gen_tcp:connect/{3,4} and ssl:connect/{3,4} and
take care of connection retries for each configured family.