daemon: Fix read-file so it fails gracefully for large files (RHBZ#589039).

daemon: Fix read-file so it fails gracefully for large files (RHBZ#589039).

Pengzhen Cao noticed that read-file would fail for files
larger than the protocol size; this is *not* the bug.  However
it would also lose protocol synchronization after this.

The reason was that functions which return RBufferOut in the
generator must not 'touch' the *size_r parameter along error
return paths.

I fixed read-file and initrd-cat, and I checked that pread was
doing the right thing.

This also adds regression tests for read-file with various categories
of large file.