Build fixes, container support, and usability improvements

This release includes:

• Build fixes
  • Fix build on 32-bit platforms (#221)
  • Fix build with gcc 10 (#224)
• Allow fscrypt to work in containers (#213)
• Usability improvements
  • Improve many error messages and suggestions (#219). For example, if the encrypt feature flag needs to be enabled on an ext4 filesystem, fscrypt will now show the tune2fs command to run.
  • Document how to securely use login protectors (#223), and link to that documentation when creating a new login protector (#225).
  • Try to detect incomplete locking of v1-encrypted directory (#217)
  • Several other small improvements.
• [Developers] Added command-line interface tests (#218)