Variable renaming in dbcommands.c

Variable renaming in dbcommands.c

There were several sets of very similar local variable names, such as
"downer" and "dbowner", which was very confusing and error-prone.
Rename the former to "ownerEl" and so on, similar to collationcmds.c
and typecmds.c.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://www.postgresql.org/message-id/flat/e5bce225-ee04-40c7-a280-ea7214318048%40eisentraut.org