cmd/fscrypt: add in status and purge commands

cmd/fscrypt: add in status and purge commands

This commit adds in the status command, which has 2 functions, allowing
the user to query the state of the entire system or a specific
filesystem.

This commit also adds in the purge command to remove all policy keys
corresponding to a filesystem. This (along with getting the unlock
status for the status commands) uses additional keyctl functionality in
the crypto and actions packages.

Change-Id: Ic8e097b335c044c0b91973eff19753f363f4525d