MFC: r264565

Do not set M_BESTFIT if a strategy has already been provided. This
fixes problems when using M_FIRSTFIT.

MFC: r280805

Add four new DDB commands to display vmem(9) statistics.

In particular, such DDB commands were added:

show vmem <addr>
show all vmem
show vmemdump <addr>
show all vmemdump

As possible usage, that allows to see KVA usage and fragmentation.

Approved by: re (gjb)