maint: add some of my maintainer-specific scripts

They are likely not general enough for widespread use, but they
are useful nonetheless.

In the best-case scenario, they will start to be used by other
people, and thus accordingly improved and made more general and
flexible.

In the worst case scenario, well, I still get to keep them in a
centralized, blessed place, simplifying the deployment and use
of them; so still a win for me :-)

* maint/am-ft: New script.
* maint/am-xft: Likewise.
* maint/rename-tests: Likewise.
* Makefile.am (EXTRA_DIST): Add them.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>