Factor out time keeping code into its own module: mono_time.c.

It turns out, `unix_time` is also monotonic, and is used as such, so I've
renamed the new functions to `mono_time_*`.

2018-07-08:
```
00:01 <@irungentoo> the idea used to be that the unix_time() function
  could go backward in time but I think I might have started using it like
  if it could not after I changed it so that it would never go back in time
```

1 files changed, 7 insertions, 0 deletions

diff --git a/toxcore/tox.c b/toxcore/tox.c
index c121eb2e..4c764aeb 100644
--- a/toxcore/tox.c
+++ b/toxcore/tox.c
@@ -518,6 +518,11 @@ static void set_friend_error(int32_t ret, Tox_Err_Friend_Add *error)
         case FAERR_NOMEM:
             SET_ERROR_PARAMETER(error, TOX_ERR_FRIEND_ADD_MALLOC);
             break;
+
+        default:
+            /* can't happen */
+            assert(!"impossible: unknown friend-add error");
+            break;
     }
 }
 
@@ -826,7 +831,9 @@ static void set_message_error(int ret, Tox_Err_Friend_Send_Message *error)
             break;
 
         case -5:
+        default:
             /* can't happen */
+            assert(!"impossible: unknown send-message error");
             break;
     }
 }