ZTS: Provide for nested cleanup routines

Shared test library functions lack a simple way to ensure proper
cleanup in the event of a failure.  The `log_onexit` cleanup pattern
cannot be used in library functions because it uses one global
variable to store the cleanup command.

An example of where this is a serious issue is when a tunable that
artifically stalls kernel progress gets activated and then some check
fails.  Unless the caller knows about the tunable and sets it back,
the system will be left in a bad state.

To solve this problem, turn the global cleanup variable into a stack.
Provide push and pop functions to add additional cleanup steps and
remove them after it is safe again.

The first use of this new functionality is in attempt_during_removal,
which sets REMOVAL_SUSPEND_PROGRESS.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: John Kennedy <[email protected]>
Signed-off-by: Ryan Moeller <[email protected]>
Closes #10080 

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