app: Add SO_KEEPOPEN

Add nRF91x1 specific SO_KEEPOPEN. If set, socket is kept open
when entering flight mode (+CFUN=4).

Signed-off-by: Markus Lassila <markus.lassila@nordicsemi.no>

Author: MarkusLassila

app/src/sm_at_socket.c

@@ -747,6 +747,10 @@ static int at_sockopt_to_sockopt(enum at_sockopt at_option, int *level, int *opt
 		*level = IPPROTO_IPV6;
 		*option = SO_IPV6_ECHO_REPLY;
 		break;
+	case AT_SO_KEEPOPEN:
+		*level = SOL_SOCKET;
+		*option = SO_KEEPOPEN;
+		break;
 	case AT_SO_IPV6_DELAYED_ADDR_REFRESH:
 		*level = IPPROTO_IPV6;
 		*option = SO_IPV6_DELAYED_ADDR_REFRESH;

app/src/sm_sockopt.h

@@ -23,6 +23,7 @@ enum at_sockopt {
 	AT_SO_SILENCE_ALL = 30,
 	AT_SO_IP_ECHO_REPLY = 31,
 	AT_SO_IPV6_ECHO_REPLY = 32,
+	AT_SO_KEEPOPEN = 34,
 	AT_SO_BINDTOPDN = 40,
 	AT_SO_TCP_SRV_SESSTIMEO = 55,
 	AT_SO_RAI = 61,

doc/app/at_socket.rst

@@ -477,6 +477,11 @@ Syntax
     * ``<value>`` is an integer that indicates whether ICMP echo replies for IPv6 are enabled.
       It is ``0`` for disabled or ``1`` for enabled.
 
+  * ``34`` - ``AT_SO_KEEPOPEN``.
+
+    * ``<value>`` is an integer that indicates whether socket is kept open when the modem enters the flight mode (``+CFUN=4``).
+      It is ``0`` for disabled or ``1`` for enabled.
+
   * ``40`` - ``AT_SO_BINDTOPDN`` (set-only).
 
     * ``<value>`` is an integer that indicates the packet data network ID to bind to.