Remove the ZFS test suite's dependency on sysutils/mkfile

mkfile is so small (7.6KB) that it makes more sense to import its source
directly than to manage it through ports.

tests/sys/cddl/zfs/bin/Makefile
tests/sys/cddl/zfs/bin/mkfile.c
tests/sys/cddl/zfs/include/commands.txt
Import mkfile, add it to the build, and make it available to the ZFS
test suite.

tests/sys/cddl/zfs/tests/**/*_test.sh
Remove the "require.progs" check for mkfile

tests/sys/cddl/zfs/tests/cli_root/zfs_rename/zfs_rename_005_neg.ksh
tests/sys/cddl/zfs/tests/cli_root/zpool_import/zpool_import_missing_005_pos.ksh
tests/sys/cddl/zfs/tests/hotplug/setup.ksh
tests/sys/cddl/zfs/tests/remote/cross_endian/cross_endian_001_pos.ksh
tests/sys/cddl/zfs/tests/remote/cross_endian/setup.ksh
tests/sys/cddl/zfs/tests/rsend/rsend_009_pos.ksh
tests/sys/cddl/zfs/tests/userquota/groupspace_002_pos.ksh
tests/sys/cddl/zfs/tests/userquota/userquota_001_pos.ksh
tests/sys/cddl/zfs/tests/userquota/userquota_004_pos.ksh
Use truncate instead of mkfile to create file-backed vdevs.
Truncate is faster because it creates sparse files.

MFC after: 3 weeks
Obtained from: http://www.infolab.ne.jp/~hatanou/freebsd/mkfile
Sponsored by: Spectra Logic Corp