zpool-create.sh: allow features to be disabled

The zimport.sh script makes use of the zpool-create.sh script
to construct test pools for importing with older versions of
ZoL.  It is desirable to have a way to disable all the features
so new pools can be imported with older code.

The simplest and most flexible way to achieve this was to merge
the VERBOSE_FLAG and FORCE_FLAG in to a single ZPOOL_FLAGS
variable.  The contents of this variable will be used in the
'zpool create' allowing us to easily pass arbitrary flags.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Closes #2524
Initial attempt to document events and payloads.

In no way complete - most have been trial and error and some
deducing what they could mean. It needs more information from
someone that knows the code better. But this is a start and
it lays the basic structure for adding this additional detail.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2357
Support '-H' (scripted mode) to 'zpool get'

This functionality is already available in 'zfs get'.  Providing
it for 'zpool get' is useful and good for consistency.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #2522