Add phys_avail allocation routines.

[jeff]

Summary

This depends on https://reviews.freebsd.org/D21250

To implement a more intelligent early alloc I neded to be able to split phys_avail entries. This really could grow a little more encapsulation so there is less direct list manipulation elsewhere. The primary goal is to enable domain aware early allocation but I also think a little abstraction is nicer.

Some of this code came from Drew at netflix.

[gallatin]

sys/vm/vm_phys.c

1635

Style nit .. I think these locals should be sorted alphabetically

1699

In the numa case, would it make sense to iterate to another domain rather than panic?

[kib]

sys/vm/vm_phys.c

1604

phys_avail[i + 1] != 0

1642

dot at the end of the sentence

1668

Again, end the sentence with dot.

1672

!= 0

[jeff]

sys/vm/vm_phys.c

1699

I think that the startup memory we're allocating should be so small relative to domain memory that if we run into this it indicates another bug.