backup.c: Send the cryptographic session data for all files. This includes zero-lengt...

backup.c: Send the cryptographic session data for all files. This includes zero-length files with non-zero-length resource forks.
restore.c: Allocate the cipher decryption context on-demand, thus disabling decryption / allocation of cipher context for zero-length files.

Fixes: #838
Regression tests passed:
  - compressed-encrypt-test
  - data-encrypt-test
  - encrypt-bug-test

git-svn-id: https://bacula.svn.sourceforge.net/svnroot/bacula/trunk@4663 91ce42f0-d328-0410-95d8-f526ca767f89