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/toxcore/Makefile.inc b/toxcore/Makefile.inc
index 4a007a66..a68b2a7d 100644
--- a/toxcore/Makefile.inc
+++ b/toxcore/Makefile.inc
@@ -8,6 +8,8 @@ libtoxcore_la_includedir = $(includedir)/tox
 libtoxcore_la_SOURCES = ../toxcore/ccompat.h \
                         ../toxcore/DHT.h \
                         ../toxcore/DHT.c \
+                        ../toxcore/mono_time.h \
+                        ../toxcore/mono_time.c \
                         ../toxcore/network.h \
                         ../toxcore/network.c \
                         ../toxcore/crypto_core.h \