linuxulator: handle V3 capget/capset

Linux 2.6.26 introduced 64-bit capability sets. Extend our stub
implementation to handle both 32- and 64-bit. (We still report no
capabilities in capget, and disallow any in capset.)

Reviewed by: chuck
Sponsored by: Turing Robotic Industries Inc.
Differential Revision: https://reviews.freebsd.org/D15887