[kernel-switcher] initial kernel-switcher refactoring

Migrate kernel-switcher to a separate directory and start splitting
the code into library <-> application.
[kernel-switcher] complete the separation between the library and the application
[kernel-switcher] tweak switch_kernel's installer callable expected signature
[kernel-switcher] move kswitch to entropy lib, kernel-switcher back to equo
[kernel-switcher] force superuser access
[kernel-switcher] turn the kernel switcher library into an OO interface