Revert "tree-wide: use pid_is_valid() at more places"

Revert "tree-wide: use pid_is_valid() at more places"

This reverts commit ee043777be58251e7441b4f04594e9e3792d7fb2.

It broke almost everywhere it touched.  The places that
handn't been converted, were mostly followed by special
handling for the invalid PID `0`.  That explains why they
tested for `pid < 0` instead of `pid <= 0`.

I think that one was the first commit I reviewed, heh.