Provide new tunable hw.nvme.verbose_cmd_dump

The nvme drive dumps only the most relevant details about a command when it
fails. However, there are times this is not sufficient (such as debugging weird
issues for a new drive with a vendor). Setting hw.nvme.verbose_cmd_dump=1
in loader.conf will enable more complete debugging information about each
command that fails.

Reviewed by: rpokala
Sponsored by: Netflix
Differential Version: https://reviews.freebsd.org/D20988