Variable renaming in dbcommands.c

In dbcommands.c function createdb(), there are several sets of very 
similar local variable names, such as "downer" and "dbowner", which is 
very confusing and error-prone.  The first set are the DefElem nodes 
from the parser, the second set are the local variables with the values 
extracted from them.  This patch renames the former to "ownerEl" and so 
on, similar to collationcmds.c and typecmds.c, to improve clarity.