contrib: Streamline the wget options for downloading.

This brings wget performance up to par with that of curl, which makes
downloading the contrib sources via wget about a ~6X faster.  Wget is
now also made the preferred downloader, as it's a smaller than curl,
and is GNU.

* contrib/src/main.mak (download): Remove the -p option, not suitable
for single file downloads (it's meant for retrieving all the resources
of a web page).  This fixes a warning too.  Remove the --passive
option, which is not documented.  It's probably equivalent to
--passive-ftp, which is the default anyway.  Use --waitretry instead
of -w.  The later was causing the 10 s delay to be inserted on every
HTTP redirection, wasting time.

Change-Id: Ibcd6805c66da3fb0c3f37b7128a34cf6c269f400