Add support for syscall::*fork:return probes in both parent and child.

[gnn]

Summary

This supplants the previous attempt and goes in for a modification of the fork_return() code itself.

[rwatson]

How does this play out with non-native ABIs (e.g., the Linux emulator) -- I thought SYS_fork (etc) were ABI-specific system-call numbers?

(For that matter: how does the systrace provider work with non-native ABIs generally?)

[gnn]

Remove filtering based on system call type (suggested by rwatson)