Add support for rrd restore to explicitly end xml files when using
the pipe interface by using the "-" special filename and using
ctrl-Z/newline as an end-of-file indicator.
Fix double-free bug
Avoid endless looping in case of XML errors.
suppress error message from libxml when reading XML from stdin.
This avoids breakage of the pipe protocol in case of errors.
Merge pull request #446 from stamfest/pipe-restore

Pipe restore