Enable multi-process mode in the NetBSD native target.

This enables proper support for multiple inferiors and ptrace(2)
assisted management of the inferior processes and their threads.

(gdb) info inferior
  Num  Description       Connection           Executable
* 1    process 14952     1 (native)           /usr/bin/dig
  2    <null>            1 (native)
  3    process 25684     1 (native)           /bin/ls
  4    <null>            1 (native)           /bin/ls

Without this patch, additional inferiors can be added, but not
properly controlled.

gdb/ChangeLog:

        * nbsd-nat.h (nbsd_nat_target::supports_multi_process): New
        declaration.
        * nbsd-nat.c (nbsd_nat_target::supports_multi_process): New
        function.

gdb/ChangeLog

@@ -1,3 +1,10 @@
+2020-07-21  Kamil Rytarowski  <n54@gmx.com>
+
+	* nbsd-nat.h (nbsd_nat_target::supports_multi_process): New
+	declaration.
+	* nbsd-nat.c (nbsd_nat_target::supports_multi_process): New
+	function.
+
 2020-07-20  John Baldwin  <jhb@FreeBSD.org>
 
 	* fbsd-tdep.c (fbsd_skip_solib_resolver): New function.

gdb/nbsd-nat.c

@@ -837,3 +837,11 @@ nbsd_nat_target::set_syscall_catchpoint (int pid, bool needed,
      are filtered by GDB rather than the kernel.  */
   return 0;
 }
+
+/* Implement the "supports_multi_process" target_ops method. */
+
+bool
+nbsd_nat_target::supports_multi_process ()
+{
+  return true;
+}

gdb/nbsd-nat.h

@@ -48,6 +48,7 @@ struct nbsd_nat_target : public inf_ptrace_target
 			      gdb::array_view<const int> syscall_counts)
     override;
 
+  bool supports_multi_process () override;
 };
 
 #endif /* nbsd-nat.h */