Correct the order of arguments to call pgxc_is_join_shippable() through
pgxc_FQS_find_datanodes_recurse(). Add tests to xc_FQS_join.sql to check the
fix.
This commit fixes three issues Mason Sharp reported.

1. Extra character in a shell script generated by datanode_cmd.c
2. Invalid gtm_ctl option generated by gtm_cmd.c
3. Incorrect gtm_proxy selection in datanode_cmd.c.

Appreciate for the report.