Feature/refactor datetime os dependency (#63)

* remove dependency on OS for time functions.

* add datetime epoch to and from seconds (yikes! 64-bit!). Add symmetric midnight seconds for unit test. Add unit tests.

* clean up BACnet date time warnings

* fix BACnet datetime warnings

Co-authored-by: Steve Karg <skarg@users.sourceforge.net>

src/bacnet/datetime.h

@@ -149,6 +149,11 @@ extern "C" {
         uint16_t year,
         uint8_t month,
         uint8_t day);
+
+    BACNET_STACK_EXPORT
+    void datetime_seconds_since_midnight_into_time(
+        uint32_t seconds,
+        BACNET_TIME *btime);
     BACNET_STACK_EXPORT
     uint32_t datetime_seconds_since_midnight(
         BACNET_TIME * btime);
@@ -208,6 +213,14 @@ extern "C" {
         BACNET_DATE_TIME * bdatetime,
         int32_t minutes);
 
+    BACNET_STACK_EXPORT
+    uint64_t datetime_seconds_since_epoch(
+        BACNET_DATE_TIME * bdatetime);
+    BACNET_STACK_EXPORT
+    void datetime_since_epoch_seconds(
+        BACNET_DATE_TIME * bdatetime,
+        uint64_t seconds);
+
     /* date and time wildcards */
     BACNET_STACK_EXPORT
     bool datetime_wildcard_year(