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