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

cmd/zdb/zdb.c

@@ -3059,12 +3059,22 @@ main(int argc, char **argv)
 	nvlist_t *policy = NULL;
 	uint64_t max_txg = UINT64_MAX;
 	int rewind = ZPOOL_NEVER_REWIND;
+	char *spa_config_path_env;
 
 	(void) setrlimit(RLIMIT_NOFILE, &rl);
 	(void) enable_extended_FILE_stdio(-1, -1);
 
 	dprintf_setup(&argc, argv);
 
+	/*
+	 * If there is an environment variable SPA_CONFIG_PATH it overrides
+	 * default spa_config_path setting. If -U flag is specified it will
+	 * override this environment variable settings once again.
+	 */
+	spa_config_path_env = getenv("SPA_CONFIG_PATH");
+	if (spa_config_path_env != NULL)
+		spa_config_path = spa_config_path_env;
+
 	while ((c = getopt(argc, argv, "bcdhilmM:suCDRSAFLXevp:t:U:P")) != -1) {
 		switch (c) {
 		case 'b':