Send a git-annex user-agent when downloading urls.

Overridable with --user-agent option.

Not yet done for S3 or WebDAV due to limitations of libraries used --
nether allows a user-agent header to be specified.

This commit sponsored by Michael Zehrer.

doc/bugs/importfeed_fails_when_using_the_option_--lazy_for_specific_podcast.mdwn

@@ -71,3 +71,7 @@ HTTP request sent, awaiting response... 200 OK
 > switch, and/or to make git-annex set a default user agent header
 > of "git-annex", rather than relying on the curl/wget defaults.
 > --[[Joey]] 
+
+> I've [[done]] what's discussed above, and verified it fixes
+> behavior for this specific server too.
+> --[[Joey]] 

doc/git-annex.mdwn

@@ -824,6 +824,10 @@ subdirectories).
 
   Also, '\\n' is a newline, '\\000' is a NULL, etc.
 
+* `--user-agent=value`
+
+  Overrides the User-Agent to use when downloading files from the web.
+
 * `-c name=value`
 
   Used to override git configuration settings. May be specified multiple times.