swapoff(2): replace special device name argument with a structure

For compatibility, add a placeholder pointer to the start of the
added struct swapoff_new_args, and use it to distinguish old vs. new
style of syscall invocation.

Reviewed by: markj
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33165