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, 2 insertions, 0 deletions

diff --git a/auto_tests/toxav_many_test.c b/auto_tests/toxav_many_test.c
index e89a6823..94183421 100644
--- a/auto_tests/toxav_many_test.c
+++ b/auto_tests/toxav_many_test.c
@@ -176,6 +176,8 @@ static void *call_thread(void *pd)
 
     printf("Closing thread\n");
     pthread_exit(nullptr);
+
+    return nullptr;
 }
 
 static void test_av_three_calls(void)