Enhancements to zpool upgrade processing

Introduce a seperate phase to list all unavailable pools when listing
pools to upgrade. This avoids confusing output when displaying older
and disabled feature pools. These existing phases now silently skip
unavailable pools.

Introduce cb_unavail to upgrade_cbdata_t which enables the final
output for zpool list to correctly detail if all pools or only all
available pools where up-to-date on version / features.

Correct the type of upgrade_cbdata_t.cb_first from int -> boolean_t.

Change the pool iteration when upgrading named pools to include
unavailable pools and update upgrade_one so it doesn't try to upgrade
unavailable pools but warns about them. This allows the correct error
to be displayed as well as upgrades with available and unavailable
pools intermixed to partially complete.

Also correct some missing trailing \n's from output in upgrade_one.

MFC after:	1 month
X-MFC-With:	r276194


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276226 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f