Use PR_SET_PTRACER to allow the minidumper child to attach to the parent

[sfackler]

What happened?

When running in a ptrace-restricted Linux environment, the child minidumper process isn't allowed to attach to the parent process via ptrace to perform minidump operations.

What did you want to happen?

We should use PR_SET_PTRACER when booting up the minidumper child process to allow it to trace properly.

[sfackler]

The rstack library we use for thread dump diagnostics also uses PR_SET_PTRACER with a different child process which is a bit unfortunate. Once we use minidumps for everything we can just set the minidump child as the ptracer immediately, but for now the best option is to probably call prctl in the signal handler right before we request the dump. prctl isn't explicitly documented to be async-signal-safe, but I assume (hope?) it's just a wrapper around the raw syscall so it should be fine.