Add log and download callbacks to use within a sandbox. These are
designed to be passed from the sandbox to the parent through a file
descriptor and then processed into alpm callbacks to be passed to the
frontend.
Note, only callbacks used in libalpm are added. Other callbacks should
be set to NULL in the child process.
Add alpm_sandbox_child() function that will be used for switching to a
less priviledged user to run child processes.
Signed-off-by: Allan McRae <allan@archlinux.org>
