- move native backend to pkg/proc/native
- move gdbserver backend to pkg/proc/gdbserial
- move core dumps backend to pkg/proc/core

Fixes #645

Fixes a bug on OSX where, if the debugged process spawned a child, when
that process received a SIGCHLD it would cause Delve to hang.

Fixes #197

This allows greater flexibility for using a port set and determining
whether the wait returned due to a breakpoint or process natural death.