Problem:
- The `ZR` feature makes it more obvious that we need some sort of flag so that
an `ExitPre` / `QuitPre` / `VimLeave` handler can handle restarts differently
than a normal exit. For example, it's common that users want `:mksession` on
restart, but perhaps not on a normal exit.
- Nvim has no way to report its "uptime".
Solution:
- Introduce `v:starttime`
- Introduce `v:exitreason`