cam: Use FreeBSD standard copyright

For CAM, move to the FreeBSD standard copyright rather than the 'put it
at the front' variation. This variaiton has been flagged as potentially
problematic in other contexts. Since this variation wasn't a conscious
decision on our part, use the standard license from src/COPYRIGHT.
Also, remove the -FreeBSD suffix in SPDX-License-Identifier. It's
obsolete at SPDX and even the original text didn't match it.

MFC After: 3 days
Sponsored by: Netflix