taskset: Accept 0 pid for current process

taskset: Accept 0 pid for current process

This is useful to print the current mask without using `$$`: `taskset -p 0`.

It is also helpful to test taskset: `taskset -c 1-4 taskset -p 0`.
This is not easy with `$$`.

sched_setaffinity(2)/sched_getaffinity(2) accept 0 for the calling
thread, so this seems consistent.

As an implementation detail, we replace 0 with getpid(), so the existing
pid != 0 <==> "will exec" logic continues to work unchanged.

A reasonable alternative would be to interpret just `taskset` (currently
an error) as printing the current mask.  This seems less orthogonal,
and a better use may be found for plain `taskset` in the future.

Signed-off-by: Jesse Rosenstock <jmr@google.com>