Add support for passing in a mutex. If NULL is passed a global
subr_unit mutex is used.
Add alloc_unrl() which expects the mutex to be held.
Allocating a unit will never sleep as it does not need to allocate
memory.
Cut possible range in half so we can use -1 to mean "out of number".
Collapse first and last runs into the head by means of counters.
This saves memory in the common case(s).
Allocation is always lowest free unit number.
A mixed range/bitmap strategy for maximum memory efficiency. In
the typical case where no unit numbers are freed total memory usage
is 56 bytes on i386.
malloc is called M_WAITOK but no locking is provided (yet). A bit of
experience will be necessary to determine the best strategy. Hopefully
a "caller provides locking" strategy can be maintained, but that may
require use of M_NOWAIT allocation and failure handling.
A userland test driver is included.
