Add zpool_clear_005_pos

It tests that "zpool clear" can reactivate a suspended pool after all vdevs
have disappeared and then returned. It's nearly identical to
zfsd_replace_002_pos, but tests manual reactivation rather than automatic
reactivation.

tests/sys/cddl/zfs/tests/cli_root/zpool_clear/Makefile
tests/sys/cddl/zfs/tests/cli_root/zpool_clear/zpool_clear_005_pos.ksh
tests/sys/cddl/zfs/tests/cli_root/zpool_clear/zpool_clear_test.sh
Add the new test

tests/sys/cddl/zfs/tests/zfsd/zfsd_replace_002_pos.ksh
tests/sys/cddl/zfs/tests/zfsd/zfsd_test.sh
Improve some test descriptions

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