Add device and ifnet logging methods, similar to device_printf / if_printf

• device_printf() is effectively a printf
• if_printf() is effectively a LOG_INFO

This allows subsystems to log device/netif stuff using different log levels,
rather than having to invent their own way to prefix unit/netif names.

Differential Revision: https://reviews.freebsd.org/D29320
Reviewed by: imp