Release files use different field names for checksum fields and the
set of allowed filenames is larger (slashes are allowed). This patch
allows switching the regexp used to validate filenames and the fields
for checksums to allow using parse_file_list not only for .dsc files,
but also for Release files.

From a reject exception:

|   File "/srv/ftp-master.debian.org/dak/dak/daklib/upload.py", line 155, in check
|     if e.errno == errno.ENOENT:
| NameError: global name 'errno' is not defined

A custom exception is nicer to handle in the caller.

Look in signature_history table if the changes was already seen to
prevent re-uploading old changes files.

Reading the file to check once rather than three times is both faster
and shorter.
Signed-off-by: NHelmut Grohne <helmut@subdivi.de>
Signed-off-by: NHelmut Grohne <helmut@subdivi.de>

This should allow dak to reject some more invalid uploads without throwing an
unhandled exception.

The upload will still be rejected later, but process-upload will be
happier.

We have to raise an exception right away after all if "entry" is None as we
later assign to "entry['sha*sum']".

If a file is only listed in the Checksums-* fields, "entry" will be None and we
cannot call the get method to compare the size in the different fields.

This change just skips the comparison when "entry" is None. We can do this as
we later check that each entry has all required checksums (i.e. is listed in
Files and the Checksums-* fields).

Reported-by: NLaszlo Kajan <lkajan@rostlab.org>

This caused an error when rejecting an upload with an invalid .dsc.

We also do not want to load the .dsc before we have made sure it matches the
hash from the .changes.

Allow to create HashedFile, Binary and Source objects from existing files.
This will be used to directly import binary and source packages into an
archive.

This makes sure we have the correct file when accessing the source
property of a Changes object.