Eliminate the test_suites.FreeBSD.keep_pools Kyua config variable

Failing to set this variable would result in bad stuff happening, like the
ZFS test suite destroying or exporting your existing pools. Instead, modify
the test suite to always ignore every pool whose name doesn't match
/^testpool/.

tests/sys/cddl/zfs/include/libtest.kshlib

• Add other_pools_exist. It returns true if the system contains any

	  pools whose names don't match /^testpool/

• Modify all_pools to exclude pools whose names don't match

	  /^testpool/, and ignore KEEP.

tests/sys/cddl/zfs/tests/cli_root/zfs_mount/zfs_mount_009_neg.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_mount/zfs_mount_all_001_pos.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_mount/zfs_mount_test.sh
tests/sys/cddl/zfs/tests/cli_root/zfs_set/canmount_002_pos.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_set/zfs_set_test.sh
tests/sys/cddl/zfs/tests/cli_root/zfs_unmount/zfs_unmount_009_pos.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_unmount/zfs_unmount_all_001_pos.ksh
tests/sys/cddl/zfs/tests/cli_root/zfs_unmount/zfs_unmount_test.sh
tests/sys/cddl/zfs/tests/cli_root/zfs_upgrade/zfs_upgrade_test.sh
tests/sys/cddl/zfs/tests/cli_root/zpool_upgrade/zpool_upgrade_test.sh
tests/sys/cddl/zfs/tests/mount/mount_test.sh
tests/sys/cddl/zfs/tests/mount/mounttest.ksh
Conditionally skip tests based on other_pools_exist, not KEEP.

share/man/man7/tests.7
Remove keep_pools from the man page

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