outsource download retry logic to download template

instead of BUTCH_MIRROR_URL, BUTCH_MIRROR_URLS (with trailing S)
is now expanded to a space-delimited list of download urls.
the template code can iterate over it in a for loop and use custom
settings for how many times to retry or to pick a random server etc.
add -std=c99 to Makefile (needed by latest libulz)
print error message if user has outdated download template
improve warnings