Add getppid op for moarvm

[ugexe]

Allows a child process to get the parent process id, and thus guess if it has become orphaned.

[ugexe]

Requires: MoarVM/MoarVM#748

This node PR should make the js implementation simple enough once it lands.

Java 9 is adding a Process/ProcessHandle interface that should make this trivial (as well as an alternative to the jvm backends current implementation of getpid), but not sure about Java 8

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An example of child determining that the parent process is no longer running:

./install/bin/perl6 -e 'use nqp; say "Original PID: " ~ nqp::getpid(); start shell q|nqp-m -e "nqp::say(\"Parent PID pre-exit: \" ~ nqp::getppid() ~ nqp::if(nqp::sleep(5),\"\nParent PID post-exit: \") ~ nqp::getppid())"|; sleep 1;'
Original PID: 33299
Parent PID pre-exit: 33299
Parent PID post-exit: 1