Pipe restore

Hi Tobi!

This is a modification to rrdrestore when using the pipe interface. The current problem is, that the pipe interface is not really designed to read an XML file from stdin. This patch corrects this. It does introduce a slight (backwards compatible) change wrt eof detection by waiting for EITHER proper eof on stdin or the sequence CTRL-Z NEWLINE (that is ascii code 26 followed by newline). Stdin here gets denoted by using "-" as the input filename.

Please fetch/merge branch pipe-restore to receive on changeset for this change, including modifications to the man page.