Remove all references to "xlog" from SQL-callable functions in pg_proc.

Commit f82ec32a renamed the pg_xlog
directory to pg_wal.  To make things consistent, and because "xlog" is
terrible terminology for either "transaction log" or "write-ahead log"
rename all SQL-callable functions that contain "xlog" in the name to
instead contain "wal".  (Note that this may pose an upgrade hazard for
some users.)

Similarly, rename the xlog_position argument of the functions that
create slots to be called wal_position.

Discussion: https://www.postgresql.org/message-id/CA+Tgmob=YmA=H3DbW1YuOXnFVgBheRmyDkWcD9M8f=5bGWYEoQ@mail.gmail.com