Hello,

Please review.

commit 5446f5e7983ad9383e3bc0469327c94baa5e0587 (HEAD -> netbsd-3)
Author: Kamil Rytarowski <n54@gmx.com>
Date:   Thu Apr 30 16:13:33 2020 +0200

    Add support for NetBSD thread events (create, exit)

    Report LWP CREATE and LWP EXIT events and setup this on post_attach()
    and post_startup_inferior().

    Stop reinitializing the list of recognized threads in update_thread_list().

    Handle LWP CREATE and EXIT events in nbsd_nat_target::wait().

    gdb/ChangeLog:

            * nbsd-nat.c (nbsd_enable_proc_events)
            (nbsd_nat_target::post_startup_inferior): Add.
            (nbsd_nat_target::post_attach): Call `nbsd_enable_proc_events'.
            (nbsd_nat_target::update_thread_list): Rewrite.
            (nbsd_nat_target::wait): Handle "PTRACE_LWP_EXIT" and
            "PTRACE_LWP_CREATE".
            * nbsd-nat.h (nbsd_nat_target::post_startup_inferior): Add.