Add argv_array_detach and argv_array_free_detached

Add argv_array_detach and argv_array_free_detached

Allow detaching of ownership of the argv_array's contents and add a
function to free those detached argv_arrays later.

This makes it possible to use argv_array efficiently with the exiting
struct child_process which only contains a member char **argv.

Add to documentation.

Signed-off-by: Florian Achleitner <redacted>
Acked-by: David Michael Barr <redacted>
Signed-off-by: Junio C Hamano <redacted>