gsignal() leaks the pgfind() reference — permanent pgrp/session leak if ever called

Summary

gsignal calls pgfind(pgid) which returns a held reference that must be released with pgrel (documented kern_proc.c:694-698; every other caller pairs them) and never pgrel()s, while pgsignal takes and drops its own separate reference — each call leaks one pg_refs and the pgrp plus its session reference can never be reclaimed. No in-tree callers today (dead exported symbol): latent leak filed so the symbol is fixed before someone wires it up. pgrel-after-pgsignal fix proposed.