Override default SPA config location via environment

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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1545