[lldb] [Process/NetBSD] Copy watchpoints to newly-created threads

NetBSD ptrace interface does not populate watchpoints to newly-created
threads.  Solve this via copying the watchpoints from the current thread
when new thread is reported via TRAP_LWP.

Add a test that verifies that when the user does not have permissions
to set watchpoints on NetBSD, the 'watchpoint set' errors out gracefully
and thread monitoring does not crash on being unable to copy watchpoints
to new threads.

Differential Revision: https://reviews.llvm.org/D70023