NVME trim stuff.

Add two sysctls to control pacing of nvme
trims. kern.cam.nda.X.goal_trim is the number of upper layer
BIO_DEELETE requests to try to collecet before sending TRIM down too
the nvme drive. trim_ticks is the number of ticks, at mosot, to wait
for at least goal_trim BIOS_DELEETE requests to come in.

Trim pacing is useful when a large number off disjoint trims are
comoing in from the upper layers. Since we have no way to chain
toogether trims from the upper layers that are sent down, this acts as
a hueristic to group trims into reasonable sized chunks. What's
reasonable varies from drive to drive.

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