Checksum verification improvements.
A good amount of changes.

actions._install() has been considerably improved. It is now a much
safer operation, and uses the new SHA-1 checksum code for verifying
package archives.

the 'actions' module also received some improvements, notably
documentation and testability-related ones. all functions return a value
now, regardless of raising exceptions or not in case of failure.

the 'archive' module received a small improvement related to file
handling.