* devices.h (device::exists_func): Redefine field.

* devices.h (device::exists_func): Redefine field.
(device::dev_on_fs): Remove unneeded bit field.  Just make a normal boolean.
(device::exists): Redefine function.
* devices.in: Move previous functions earlier since they are now only defined
static.  Rename some functions due to an as-yet unresolved bug in gendevices.
Rename posix part of internal-only devices with a double-slash.  gendevices
will eventuall translate that to a ":".
(device::parse): Rework to use dev pointer and dev_storage_end.
* devices.cc: Regenerate.
* gendevices: Translate "// to ": after shilka processing.