[lldb] [Process/FreeBSDRemote] Explicitly copy dbregs to new threads

Explicitly copy dbregs to new threads to ensure that watchpoints
are propagated properly.  Fixes the test failure due to apparent kernel
race between reporting a new thread and resuming main thread execution
that makes implicit inheritance of dbregs unreliable.  By copying them
explicitly, we ensure that the new thread correctly respects watchpoints
that were set after the thread was created but before it was reported.

The code is copied from the NetBSD plugin and modernized to use
llvm::Error.

Differential Revision: https://reviews.llvm.org/D91032