process: Provide an abstraction to spawn child processes with redirected I/O
process: Port child process spawning to the Windows platform
process: Add a wrapper to invoke a command under the system default shell
updown: Use process abstraction to invoke updown script
ext-auth: Add an ext-auth plugin invoking an external authorization script

Original patch courtesy of Vyronas Tsingaras.
NEWS: Mention new ext-auth plugin
Merge branch 'ext-auth'

Integrates the ext-auth plugin by Vyronas Tsingaras. The new child process
abstraction simplifies implementation in both the new ext-auth and the existing
updown plugin, and makes them available on the Windows platform.