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/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;
+}