um: clean Kconfig up a bit

* kill duplicates with drivers/char/Kconfig
* take watchdog one into drivers/watchdog/Kconfig
* take mmapper to arch/um/Kconfig.um
* rename Kconfig.char menu to "UML Character Devices"

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>