wget on RHEL6 is too old to implement --method=HEAD.  curl works
just fine.

In some places we were using the test_curl_HEAD() function, in others
we were manually calling it.  Instead, use the helper function
everywhere.

Add --location flag to curl command so that if S3 returns us a 302
redirect (which it may do for its own purposes), we follow it properly.

This adds a function similar to the shell 'which' command to detect
if curl is available, rather than assuming curl is available on
a posix system.  Cleaner.