ptrace: Allow tracing programs started with system() or fork+exec

[SimonKagstrom]

Useful e.g., where a wrapper is used to start programs (but the wrapper itself is uninteresting).

Could perhaps be implemented through using LD_PRELOAD and overriding exec().

[SimonKagstrom]

Not as easy as I first thought: Overriding exec in LD_PRELOAD means recursive ptrace, so that won't work.

Can probably be made to work by starting the process from kcov itself instead, but that's a major change.