MFV r303082: 6391 Override default SPA config location via environment

illumos/illumos-gate@ae24175b2b25e9fb4bfd9ac0648b57e0735b6651
https://github.com/illumos/illumos-gate/commit/ae24175b2b25e9fb4bfd9ac0648b57e0735b6651

https://www.illumos.org/issues/6391

When using zdb with non-default SPA config file it is not convenient
to add -U <non-default-config-file-path> all the time. This commit
introduces support for setting/overriding SPA config location via
environment variable 'SPA_CONFIG_PATH'.
If -U flag is specified in the command line it will override any other
value as usual.

https://github.com/zfsonlinux/zfs/commit/64d7b6cf75e52a4698d9bdec61745573c39d2e5a

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Richard Yao <ryao@gentoo.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Cyril Plisko <cyril.plisko@mountall.com>
MFC after: 1 week