nspawn: decouple --boot from CLONE_NEWIPC (#4180)

nspawn: decouple --boot from CLONE_NEWIPC (#4180)

This commit is a minor tweak after the split of `--share-system`, decoupling the `--boot`
option from IPC namespacing.

Historically there has been a single `--share-system` option for sharing IPC/PID/UTS with the
host, which was incompatible with boot/pid1 mode. After the split, it is now possible to express
the requirements with better granularity.

For reference, this is a followup to #4023 which contains references to previous discussions.
I realized too late that CLONE_NEWIPC is not strictly needed for boot mode.