Allow copydir() to be interrupted.

Allow copydir() to be interrupted.

This makes ALTER DATABASE .. SET TABLESPACE and CREATE DATABASE more
sensitive to interrupts.  Backpatch to 8.4, where ALTER DATABASE .. SET
TABLESPACE was introduced.  We could go back further, but in the absence
of complaints about the CREATE DATABASE case it doesn't seem worth it.

Guillaume Lelarge, with a small correction by me.