Generalise the new background daemon code

Generalise the new background daemon code

Move the new background daemon code out from under the AFS_DARWIN80_ENV
ifdef into its own AFS_NEW_BKG define. This lets us use it for things
other than the Darwin mv helper, on other platforms.

Based on code originally written by Derrick Brashear

Change-Id: Iba9431f5dc4bbbe10222b25a595cdd1ea2f592f4
Reviewed-on: http://gerrit.openafs.org/7136
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>