layer23: Use osmo_strlcpy() to avoid non-terminated strings

layer23: Use osmo_strlcpy() to avoid non-terminated strings

settings.c:191:2: warning: ‘strncpy’ output may be truncated copying 15 bytes from a string of length 15 -Wstringop-truncation]
  strncpy(set->imeisv, set->imei, 15);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  CC       subscriber.o
  CC       support.o
  CC       transaction.o
  CC       vty_interface.o
  CC       voice.o
  CC       mncc_sock.o
  CC       primitives.o
mncc_sock.c: In function ‘osmo_unixsock_listen’:
mncc_sock.c:318:2: warning: ‘strncpy’ specified bound 108 equals destination size [-Wstringop-truncation]
  strncpy(local.sun_path, path, sizeof(local.sun_path));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  CC       script_lua.o
vty_interface.c: In function ‘cfg_gps_device’:
vty_interface.c:1144:2: warning: ‘strncpy’ specified bound 32 equals destination size [-Wstringop-truncation]
  strncpy(g.device, argv[0], sizeof(g.device));
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  AR       libmobile.a

Change-Id: Id52978f3bf7a8abea62237d7c32f8f87e1bb34a1