Some sites, most notably github redirecting to amazon s3 storage and
googlecode, refuse HEAD requests (403). For those, fallback to a GET request
with a 'Range: bytes 0-0' header.

Other sites refuse curl user agent (403). Because of those, one now send a
'robotpkg' user agent (although it's still curl, but what else can be done?).

While here, add a variable NO_MASTER_SITES_CHECK to disable the
check-msster-sites target for a package.