Replace the remote/[script_name]/exec option with remote_exec

The remote/[script_name]/exec option is replaced by the remote_exec
option. With the addition of steps configuration, it becomes useless
to have per step exec option.

Also rename remote_ssh/exec and remote_chroot/exec to remote_ssh and
remote_chroot.
Create zip files silently
doc: add remote_exec in the list of options
Add the suexec option
Add support for pre and post scripts

Those scripts are run as root before and after a build. They can be used
to install/remove build dependencies.