Core Updates: Verify that all files we're about to copy are actually writable, when using the direct transport.

Once we begin to copy core files, all bets are off. This allows us to fail early when we'll otherwise need to stop midway through due to permissions issues, which is a particularly bad problem when only some files have permissions issues.

see #22704.

Built from https://develop.svn.wordpress.org/trunk@25794


git-svn-id: http://core.svn.wordpress.org/trunk@25706 1a063a9b-81f0-0310-95a4-ce76da25c4cd