Rename struct device to struct _device

types.h defines device_t as a typedef of struct device *. struct device
is defined in subr_bus.c and almost all of the kernel uses device_t.
The LinuxKPI also defines a struct device, so type confusion can occur.

This causes bugs and ambiguity for debugging tools. Rename the FreeBSD
struct device to struct _device.

Reviewed by: gbe (man pages)
Reviewed by: rpokala, imp, jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29676

(cherry picked from commit dfff37765ce4ea4fd7db4d293b459dc84008f411)