Background Updates: Introduce support to take advantage of Group Writable (or World Writable) to Core Background updates.
This is only enabled when new files will not be installed during the update (as indicated by the WordPress.org API), and does not apply to Plugin/Theme/Translation Background Updates.

Additionally, the code to determine if the 'direct' filesystem transport should be used has been tweaked for wider support (where getmyuid() was unavailalbe) which fixes #10424

See #10205, #30245


git-svn-id: https://develop.svn.wordpress.org/trunk@30384 602fd350-edb4-49c9-b593-d223f7449a82