Copy collencoding in CREATE COLLATION / FROM

Copy collencoding in CREATE COLLATION / FROM

This command used to compute the collencoding entry like when a
completely new collation is created.  But for example when copying the
"C" collation, this would then result in a collation that has a
collencoding entry for the current database encoding rather than -1,
thus not making an exact copy.  This has probably no practical impact,
but making this change keeps the catalog contents neat.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>