cpuset: Fix sched_[g|s]etaffinity() for better compatibility with Linux.

Under Linux to sched_[g|s]etaffinity() functions the value returned from a call
to gettid(2) (thread id) can be passed in the argument pid. Specifying pid as 0
will set the attribute for the calling thread, and passing the value returned
from a call to getpid(2) (process id) will set the attribute for the main thread
of the thread group.

Native cpuset(2) family of system calls has "which" argument to determine how
the value of id argument is interpreted, i.e., CPU_WHICH_TID is used to pass
a thread id and CPU_WHICH_PID - to pass a process id.

For now native sched_[g|s]etaffinity() implementation is wrong as uses "which"
CPU_WHICH_PID to pass both (process and thread id) to the kernel. To fix this
adding a new "which" CPU_WHICH_TIDPID intended to handle both id's.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D38209
MFC after: 1 week