Add stub for PluginProcessNetBSD

Summary:
This is the base for introduction of further features to support Process Tracing on NetBSD, in local and remote setup.

This code is also a starting point to synchronize the development with other BSDs. Currently NetBSD is ahead and other systems can catch up.

Sponsored by <The NetBSD Foundation>

Reviewers: emaste, joerg, kettenis, labath

Reviewed By: labath

Subscribers: mgorny, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D31138

llvm-svn: 298408