askpass: signal handling and timeout

Allow SIGTERM and SIGINT after the method preparation phase and before actually printing the passphrase to standard output (so effectively while reading the passphrase).

The signals are blocked again before the passphrase is printed in order to prevent a partial passphrase, which may then be directly used for deriving a key for a plain dm-crypt device and thus could cause data corruption.

Signed-off-by: Christoph Anton Mitterer mail@christoph.anton.mitterer.name