add yosemite to macOSXSeries
Prevent destroy-environment when desired.
Fixed error construction and format. Renamed vars for clarity and conformity.
Cleaned up error-related code based on review.
Made code readable
Environment variable manipulation is behind cmds for block/unblock operation.
Code readability improvement.
state: extracted environment setup ops

The code to generated DB operations for setting up the initial
environment have been extracted so that they can also be called from
elsewhere.
Mock command for tests.
More verbose command purpose for help.
Move backupsMetaC into backups.go.
Move state/backups.go to state/backups/storage.go.
Use the common implementation for getting a new State.
Add a note about dropping the corresponding function in state.
Use *state.State rather than the temp surrogate interfaces.
Fix a couple storage names.
Gut the backup plugin, but keep it around for backward-compatibility.
Use the old plugin code for 1.20.
Add a deprecation warning message.
Add a TODO comment about removing the plugin.