Rename CONFIG_NETUTILS_DNS* to CONFIG_NETDB_DNS* to reflect the movement of the DNS server from apps/netutils to nuttx/libc/netdb

Update protected build logic for all platforms that supported the protected build.  This is necessary due to changes in the definition of the userspace interface

Remove obsoleted CONFIG_SYSTEM_POWEROFF from configurations.  The NSH shutdown command should be available instead

Enable CONFIG_BOARD_POWEROFF=y.  Now you can use the NSH shutdown command to terminate the NSH session