gpg: Make the use of "--verify FILE" for detached sigs harder.

* g10/openfile.c (open_sigfile): Factor some code out to ...
(get_matching_datafile): new function.
* g10/plaintext.c (hash_datafiles): Do not try to find matching file
in batch mode.
* g10/mainproc.c (check_sig_and_print): Print a warning if a possibly
matching data file is not used by a standard signatures.

Allowing to use the abbreviated form for detached signatures is a long
standing bug which has only been noticed by the public with the
release of 2.1.0. :-(

What we do is to remove the ability to check detached signature in
--batch using the one file abbreviated mode. This should exhibit
problems in scripts which use this insecure practice. We also print a
warning if a matching data file exists but was not considered because
the detached signature was actually a standard signature:

gpgv: Good signature from "Werner Koch (dist sig)"
gpgv: WARNING: not a detached signature; \
file 'gnupg-2.1.0.tar.bz2' was NOT verified!

We can only print a warning because it is possible that a standard
signature is indeed to be verified but by coincidence a file with a
matching name is stored alongside the standard signature.

• Reported-by: Simon Nicolussi (to gnupg-users on Nov 7)
• Signed-off-by: Werner Koch <wk@gnupg.org>