Reset some stats and a buffer when MythDownloadManager gets a redirect.

Fixes #11218.
Add a --download option to mythutil.

"--download --infile URI --outfile FILENAME" will download the file
pointed to by 'URI' using MythDownloadManager and save it as 'FILENAME'.

URI is expected to use one of http://, ftp://, or myth:// protocols
since these are the only ones currently supported by MythDownloadManager.