process-util: remove broken support for pid==0

process-util: remove broken support for pid==0

Our functions that query /proc/pid/ support using pid==0 to mean
self. get_process_id also seemed to support that, but it was not implemented
correctly: the result should be in *uid, not returned, and also it gave
completely bogus result when called from get_process_gid(). But afaict,
get_process_{uid,gid} were never called with pid==0, so it's not an actual
bug. Remove the broken code to avoid confusion.