portsnap patch to use fetch (instead of wget)

I did some review to my micro portsnap patch, thanks also to the suggestion on the FreeBSD forums and -stable mailing list.

In particular, as someone pointed out, using wget(1) is not a good idea because it is not part of the base system and requires an admin to manually install it from ports or packages. That is not a big deal in my opinion, and it is the reason why I made a change so that it was possible to configure, using /etc/portsnap.conf, a way to specify that wget(1) was required. To make it even clearer, I also created a function check_downloader that is used to check the presence of the program executable used to download stuff (either phttpget or wget).

I was not fully happy with such patch, but I have not a better idea until someone on the -stable mailing list pointed out that FreeBSD (of course!) has already a downloader: fetch(1).

The problem with fetch(1) is that it requires a full URL while portsnap was internally dealing with clean server names.

So I reverted the patch in order to avoid using wget(1) and to use fetch(1) by means of a flag in /etc/portsnap.conf and introduced a new variable in the internal portsnap implementation, SERVER_PROTOCOL, that simply handles "http" in the case of the usage of fetch(1).

The patch has moved into another path within my repository and is available here.