This is primarily to fix #2368

• Emulate PR_SET_DUMPABLE instead of allowing it to execute natively, which interferes with Shadow's ability to access managed process memory.

Some other minor improvements:

• Return an error for some other prctl operations that would interfere with Shadow
• Add explicit cases for all known prctl operations, and return errors for any others.
• Enable the getgroups syscall to just execute natively.
• Add explicit cases for the unimplemented sched_getaffinity and sched_setaffinity syscalls, so that the logged warnings give the syscall names instead of just the numbers.
• Fix clock_gettime and futex syscalls to handle the cases where a bad user pointer is provided.