Fix handling of rsync remote urls containing a username, including rsync.net.

This breakage seems to have been caused way back in a1eded86,
but I am pretty sure rsync.net support has not been entirely
broken since last April. AFAICS, the generated .ssh/config
has not changed since then -- it has never included a Username setting
line. So, I am puzzled at when this reversion was introduced.

Note that the breakage only affected checkpresent and remove. Upload and
download use the ssh connection caching, which includes a -l username.

doc/bugs/rsync_transport:_username_not_respected.mdwn

@@ -36,3 +36,8 @@ Type: prebuilt
 
 # End of transcript or log.
 """]]
+
+> Argh! How did that break? I know it used to work.
+> I have fixed it, unfortunately the fix was too late for today's release,
+> but it will be available in autobuilds shortly.
+> [[fixed|done]] --[[Joey]]