Reland "[lldb] [Process] Watch for fork/vfork notifications" for FreeBSD

The original commit was reverted because of the problems it introduced
on Linux.  However, FreeBSD should not be affected, so restore that part
and we will address Linux separately.

While at it, remove the dbreg hack as the underlying issue has been
fixed in the FreeBSD kernel and the problem is unlikely to happen
in real life use anyway.

Differential Revision: https://reviews.llvm.org/D98822