Unclean shutdown counter

[Squidly271]

Can we get emhttp to keep track of the number of unclean shutdowns detected (and the date of the last detection), and have it stored in /boot/config or something so it can be included in the diagnostics . Non-resettable from the GUI, unless a New Config is issued. Logical place for the code is in emhttp, so no PR from me here.

Might help in determining certain corruption issues, how stable the user's system actually is, etc.