Add the bus_alloc_resource_anywhere() convenience function.

Summary:
Many instances of bus_alloc_resource() simply use 0 and ~0 for start and end to
denote 'anywhere' with a given count. To clean this up, introduce a
bus_alloc_resource_anywhere() convenience function.

Bump __FreeBSD_version for the new API.

Reviewed By: jhb
Differential Revision: https://reviews.freebsd.org/D5370