xep-0092: add config option to set whether OS is revealed

Default is on. `revail.os=false` in the `connection` section of the
config can disable it.
A command to configure this will follow.

src/config/preferences.c

@@ -1758,6 +1758,7 @@ _get_group(preference_t pref)
         case PREF_CARBONS:
         case PREF_RECEIPTS_SEND:
         case PREF_RECEIPTS_REQUEST:
+        case PREF_REVEAL_OS:
         case PREF_TLS_CERTPATH:
             return PREF_GROUP_CONNECTION;
         case PREF_OTR_LOG:
@@ -1817,6 +1818,8 @@ _get_key(preference_t pref)
             return "receipts.send";
         case PREF_RECEIPTS_REQUEST:
             return "receipts.request";
+        case PREF_REVEAL_OS:
+            return "reveal.os";
         case PREF_OCCUPANTS:
             return "occupants";
         case PREF_OCCUPANTS_JID:
@@ -2056,6 +2059,7 @@ _get_default_boolean(preference_t pref)
         case PREF_BOOKMARK_INVITE:
         case PREF_ROOM_LIST_CACHE:
         case PREF_STATUSBAR_SHOW_NUMBER:
+        case PREF_REVEAL_OS:
             return TRUE;
         default:
             return FALSE;

src/config/preferences.h

@@ -63,6 +63,7 @@ typedef enum {
     PREF_CARBONS,
     PREF_RECEIPTS_SEND,
     PREF_RECEIPTS_REQUEST,
+    PREF_REVEAL_OS,
     PREF_OCCUPANTS,
     PREF_OCCUPANTS_SIZE,
     PREF_OCCUPANTS_JID,