[XrdCl] Make sure to handle non-NULL-terminated error messages correctly
[XrdCl] Make sure to release old authBuffer when reconnecting after a TTL expiration
[XrdCl] Retry after an incomplete write

This does not really change much except producing clearer error messages,
ie. "Run: [ERROR] OS Error: No space left on device" instead of
"Run: [ERROR] OS Error: Operation now in progress"
[XrdCl] Don't force a server to issue a short read when fetching last data chunk.

This works around a problem in broken proxies.
Release Notes: Update for 4.0.2