Allow setting the CHAP username/password via environment variables

so that the password will not be showing up in log where the application
logs the "filename/iscsi url" or in ps aux output.

LIBISCSI_CHAP_USERNAME and LIBISCSI_CHAP_PASSWORD environment variables are
available to set these outside of the url.
If hte username/password is ALSO set in the URL, the settings in the URL
will override the environment variables.

lib/init.c

@@ -262,6 +262,9 @@ iscsi_parse_full_url(struct iscsi_context *iscsi, const char *url)
 	}
 	portal = str;
 
+	user   = getenv("LIBISCSI_CHAP_USERNAME");
+	passwd = getenv("LIBISCSI_CHAP_PASSWORD");
+
 	tmp = index(portal, '@');
 	if (tmp != NULL) {
 		user = portal;