Simplify killing processes.

This is a branch I've been sitting on waiting for parrot-native-fd to be merged. This can now also be merged.

* Do not hook rt_sigaction or signal (32 bit).
* Old "interrupted" logic is unnecessary since parrot-native-fd branch.
* New pfs_process_cankill (formally pfs_process_raise) which encapsulates the
  logic pfs_dispatch*.cc needs for handling the kill system call.
* Remove obsolete logic in decode_write for handling SIGPIPE, which was necessary
  before parrot-native-fd when Parrot handled all tracee pipes.