Change the process limits for RLIMIT_MEMLOCK to RLIM_INFINITY when
executing :mincore_resid

The default process limits in FreeBSD is 64kB for unprivileged users,
which empirically is too low to run the :mincore_resid testcase.

Process limits are inherited, so even though the default limit for
root users is RLIM_INFINITY, the inherited limit with "sudo" with the
default login.conf will be 64kB.

Use setrlimit to set rlim_max for RLIMIT_MEMLOCK to RLIM_INFINITY to
avoid ENOMEM issues when calling mlock to wire the mmap'ed address
space.

setrlimit requires root access to increase rlim_max, so require root
privileges when running the test

Discovered when executing the tests with sudo, e.g.
"sudo kyua test -k /usr/tests/lib/libc/sys/Kyuafile mincore_test"

MFC after: 2 weeks