Refactor TUN Interface Management and Move to SDAP Layer

This commit refactors the TUN interface management by moving its creation
and configuration from the PDCP layer to the SDAP layer for UE.

Key changes include:

- Introduced `tun_generate_ue_ifname` and `tun_destroy` functions for better
  TUN interface lifecycle management.
- Added support for creating and destroying UE-specific TUN interfaces in SDAP.
- Refactored `nr_pdcp_module_init` to remove TUN-related logic and moved it
  to SDAP.
- Updated NAS and SDAP layers to handle PDU session-specific TUN interface
  creation and configuration.
- Enhanced SDAP entity management to include TUN socket handling and thread
  lifecycle.
- Updated NAS message handling to initialize default PDU session TUN
  interfaces in NOS1 mode.
- Use a single SDAP thread function for gNB and UE
- Allow noS1 mode also in SA on gNB

These changes improve modularity by isolating TUN interface management within
the SDAP layer, aligning with the 3GPP architecture, and simplifying the PDCP
layer.

Co-Authored-By: Robert Schmidt <robert.schmidt@openairinterface.org>

executables/softmodem-common.h

@@ -144,6 +144,7 @@ extern "C"
 #define EMULATE_L1          softmodem_params.emulate_l1
 #define CONTINUOUS_TX       softmodem_params.continuous_tx
 #define SYNC_REF            softmodem_params.sync_ref
+#define DEFAULT_PDU_ID      softmodem_params.default_pdu_session_id
 
 #define DEFAULT_RFCONFIG_FILE    "/usr/local/etc/syriq/ue.band7.tm1.PRB100.NR40.dat";
 
@@ -188,6 +189,7 @@ extern int usrp_tx_thread;
   {"E" ,                    CONFIG_HLP_TQFS,          PARAMFLAG_BOOL, .iptr=&softmodem_params.threequarter_fs, .defintval=0,            TYPE_INT,    0}, \
   {"imscope" ,              CONFIG_HLP_IMSCOPE,       PARAMFLAG_BOOL, .uptr=&enable_imscope,                   .defintval=0,            TYPE_UINT,   0}, \
   {"imscope-record" ,       CONFIG_HLP_IMSCOPE_RECORD,PARAMFLAG_BOOL, .uptr=&enable_imscope_record,            .defintval=0,            TYPE_UINT,   0}, \
+  {"default_pdu_id",        NULL,                     0,              .iptr=&DEFAULT_PDU_ID,                   .defintval=10,           TYPE_INT,    0}, \
 }
 // clang-format on
 
@@ -234,6 +236,7 @@ extern int usrp_tx_thread;
     { .s5 = { NULL } },                     \
     { .s5 = { NULL } },                     \
     { .s5 = { NULL } },                     \
+    { .s5 = { NULL } },                     \
 }
 // clang-format on
 
@@ -343,6 +346,7 @@ typedef struct {
   uint32_t       sync_ref;
   int no_itti;
   int threequarter_fs;
+  int default_pdu_session_id;
 } softmodem_params_t;
 
 #define IS_SA_MODE(sM_params) (!(sM_params)->phy_test && !(sM_params)->do_ra && !(sM_params)->nsa)