Revert "Avoid password prompting caused by --throw-keyids"

The implementation missed out on the fact that sometimes there will be no matching
key in the participants list (eg. git clone) and that we need to fallback to
trying all keys in the users keyring if that happens.

This reverts commit 8142818935f9b572c96fb5ab2a418c90f8b27015.
Second attempt at less prompting

See the commit message of 8726767e251fd8e4aaced6d7462d1be38c2df35a for the 
first attempt, and see blake2-ppc/git-remote-gcrypt#9 for some more discussion.

If gcrypt-participants is set, export all secret keys from participants in a temporary file and 
tell gpg to use that as keyring. If that fails, try like before, going over all the users keys.

The following limitations/questions are still present:
- secret keys written to tmp
- I don't manage to pass the temp file to PRIVDECRYPT so it can be passed to gpg
  quoted. I think this is a security hole, but I don't manage. If someone could fix that?
  I haven't tried stuff like this: http://www.yosefk.com/blog/passing-shell-script-arguments-to-a-subprocess.html
- the original code linked statements with "&&" upto isnonnull "$manifest_" || echo_die "Failed to decrypt manifest!"
  since we call PRIVDECRYPT twice this is no longer possible, but I don't know what this was
  good for and if removing it might break something. Note that the script calls set -e.