WIP: cli: add --no-safeguards

[Snaipe]

This commit adds a new option to allow bst to change existing
namespaces. This in particularly useful to perform any complex setup of
a spacetime via invoking bst multiple times. For instance:

$ bst --persist=./ns --share pid --mount tmp,/some/root/tmp,tmpfs true
$ bst --no-safeguards --share=./ns --share pid cp -r /files /some/root/tmp
$ bst --no-safeguards --share=./ns --root /some/root /bin/sh

... is a viable alternative to using a setup script if performance is
critical.

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This is a work in progress, and should not be merged yet. There are issues with the way that NIC creation is handled with no safeguards, due to the assumptions that bst makes on the way that interfaces are created & renamed.