Change fmgr.h typedefs to use original names

Change fmgr.h typedefs to use original names

fmgr.h defined some types such as fmNodePtr which is just Node *, but
it made its own types to avoid having to include various header files.
With C11, we can now instead typedef the original names without fear
of conflicts.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/10d32190-f31b-40a5-b177-11db55597355@eisentraut.org