Switch url for freebsd distfiles

FreeBSD has switched the location of their distfiles cache from ftp://ftp.freebsd.org/pub/FreeBSD/ports/distfiles/ to http://distcache.FreeBSD.org/ports-distfiles/. `grep` shows that `libhid.rb`, `sntop.rb`, and `wait_on.rb` are affected. This commit simply updates the urls.