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
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D29676