pg_upgrade: Add --copy option

pg_upgrade: Add --copy option

This option selects the default transfer mode.  Having an explicit
option is handy to make scripts and tests more explicit.  It also
makes it easier to talk about a "copy" mode rather than "the default
mode" or something like that, since until now the default mode didn't
have an externally visible name.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/50a97009-8ff9-ca4d-a0f6-6086a6775a5b%40enterprisedb.com