Make installation of native kernelspec explicit

Instead of creating a kernelspec directory matching the running
frontend, return its spec without writing to disk.

Adds a command 'ipython kernelspec install-self' to write a kernelspec
for the current version of Python.
Simplify kernel manager code
Merge pull request #6291 from takluyver/explicit-install-native-kernelspec

Make installation of native kernelspec explicit