Since Go 1.16 (which recently became the minimum supported Go version
for this project), the package io/ioutil is deprecated in favor of
equivalent functionality in the io and os packages.  staticcheck warns
about this.  Address all the warnings by switching to the non-deprecated
replacement functions.

This commit changes the error handling for the actions package to use
the error handling library github.com/pkg/errors. This means replacing
"errors" with "github.com/pkg/errors", reworking some of the error
values, and wrapping some errors with additional context.

This commit also changes the Protector/Policy API, moving most of the
package functionality into Protector or Policy methods. These types are
now "locked" when they are queried from the filesystem, and Unlock()
must be used to get their corresponding keys. Note that only certain
operations will require unlocking the keys. Certain unnecessary
functions and methods are also removed.

This CL also fixes two bugs reported by Tyler Hicks in CreateConfigFile.
CPU time is used instead of wall time, and kiB is used instead of kB.

Change-Id: I88f45659e9fe4938d148843e3289e7b6d5b698d8