Rename real routines to proc_find() and pgrp_find(), remove PFIND_* flags
and have consistent behaviour. Provide proc_find_raw() for special cases.
Fix memory leak in sysctl_proc_corename().
COMPAT_LINUX: rework ptrace() locking, minimise differences between
different versions per-arch.
Note: while this change adds some formal cosmetics for COMPAT_DARWIN and
COMPAT_IRIX - locking there is utterly broken (for ages).
Fixes PR/43176.