Add function to convert Date time from a dictionary to Epoch

- Also changed get_time_from_unix_time to get_date_time_from_unix_time to be
  consistent.

Ticket:
https://github.com/godotengine/godot/issues/4038

drivers/unix/os_unix.cpp

@@ -261,6 +261,7 @@ OS::Date OS_Unix::get_date(bool utc) const {
 	
 	return ret;
 }
+
 OS::Time OS_Unix::get_time(bool utc) const {
 	time_t t=time(NULL);
 	struct tm *lt;