Implement the M_NEXTFIT allocation strategy for vmem(9).

This is described in the vmem paper: "directs vmem to use the next free
segment after the one previously allocated." The implementation adds a
new boundary tag type, M_CURSOR, which is linked into the segment list
and precedes the segment following the previous M_NEXTFIT allocation.
The cursor is used to locate the next free segment satisfying the
allocation constraints.

This implementation isn't O(1) since busy tags aren't coalesced, and we
may potentially scan the entire segment list during an M_NEXTFIT
allocation.

Reviewed by: alc
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D17226