execute: Mount all file systems only in namespace

execute: Mount all file systems only in namespace

If Pakfire is running as an unprivileged user, we cannot call mount() in
the original namespace. However, it is difficult to spawn a new process
in a new namespace first and then perform loads of actions in there.
Embedded Pakfire would become more difficult.

At the cost of losing the option to create an environment in a
dynamically created ramdisk, we can only mount everything when we enter
the container.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>