Add compat.linux.dummy_rlimits, and disable by default.

Turns out the dummy rlimits fix prlimit(1), but break su(8)
(login-1:4.5-1ubuntu2) - although not sudo(8), for some reason.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26814