Tunable directory for zfs runtime scripts

zpool and zed place scripts in subdirectories of libexecdir. Some
distributions locate architecture independent scripts in other locations
(e.g. Debian). To avoid these paths getting out of sync, centralize the
definitions.

Build zfs-test's default.cfg by Makefile. Use the new directory
logic building tests/zfs-tests/include/default.cfg.in.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Antonio Russo <antonio.e.russo@gmail.com>
Closes #7597