Add optional on-device file hashing

[besendorf]

Summary

• ask whether files remaining on the device should be hashed, defaulting to the existing metadata-only behavior
• add -hash-files yes|no and -H so unattended acquisitions can explicitly choose the hashing behavior
• use the collector's hashing mode when requested and continue without hashes with a warning when the collector is unavailable
• document the resource impact and add coverage for option parsing, unattended requirements, and collector method selection

Closes #49