Add the '-a' option to 'zpool export'

Support exporting all imported pools in one go, using 'zpool export -a'.

This is accomplished by moving the export parts from zpool_do_export()
in to the new function zpool_export_one(). The for_each_pool() function
is used to enumerate the list of pools to be exported. Passing an argc
of 0 implies the function should be called on all pools.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes: #3203