subr_bus: introduce device_set_descf() and modify allocation logic

device_set_descf() is a printf-like version of device_set_desc().

Allocation code has been transferred from device_set_desc_internal() to
device_set_desc_copy() and device_set_descf() to avoid complicating
device_set_desc_internal(). The "copy" argument in
device_set_desc_internal() has been replaced with a flag which is set
when the description string has been allocated with M_BUS.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: imp, markj
Differential Revision: https://reviews.freebsd.org/D43370