Add a testenv wrapper script for the ZFS test suite. This makes it easier
to test changes to the test suite's common libraries by creating a ksh93
subshell to run ksh93 functions in.

tests/sys/cddl/zfs/include/constants.cfg:
tests/sys/cddl/zfs/include/default.cfg.in:

• Split default.cfg.in so that general constants live in constants.cfg.

tests/sys/cddl/zfs/include/testenv.kshlib:

• Generate the environment-dependent constants ($DISKS and $KEEP),

	  and pull in constants.cfg for the rest.

tests/sys/cddl/zfs/bin/testenv.ksh:

• The script. Detects the current path and generates $STF_SUITE.

	  Once done, invokes ksh93, reading in include/testenv.kshlib.

tests/sys/cddl/zfs/bin/Makefile:
tests/sys/cddl/zfs/include/Makefile:

• Add the new files.

Submitted by: Will
Sponsored by: Spectra Logic Corp