Honor spa_config_path for ztest '-f' option

The new spa_config_path string was lost from ztest_run_zdb() during
the onnv_141 merge.  This commit puts it back in place so the '-f'
option is properly honored.

Additionally this function had been tweaked so ztest could be run
in-tree but that broke running it when installed as a package.  I've
updated that chunk to detect where it's running and try to do the
right thing in both cases.

Closes #49

cmd/ztest/ztest.c

@@ -4901,12 +4901,16 @@ ztest_run_zdb(char *pool)
 	zdb = umem_alloc(MAXPATHLEN + MAXNAMELEN + 20, UMEM_NOFAIL);
 	zbuf = umem_alloc(1024, UMEM_NOFAIL);
 
-	/* Designed to be run exclusively in the development tree */
 	VERIFY(realpath(getexecname(), bin) != NULL);
-	strstr(bin, "/ztest/")[0] = '\0';
+	if (strncmp(bin, "/usr/sbin/ztest", 14) == 0) {
+		strcpy(bin, "/usr/sbin/zdb"); /* Installed */
+	} else {
+		strstr(bin, "/ztest/")[0] = '\0'; /* In-tree */
+		strcat(bin, "/zdb/zdb");
+	}
 
 	(void) sprintf(zdb,
-	    "%s/zdb/zdb -bcc%s%s -U /tmp/zpool.cache %s",
+	    "%s -bcc%s%s -U %s %s",
 	    bin,
 	    zopt_verbose >= 3 ? "s" : "",
 	    zopt_verbose >= 4 ? "v" : "",