timespec UPDATE reworked time functions

merged some time functions in order to be able to directly get some time in the future with a single function call
instead of having to get the current time and then add some time to it, also created a new function, which returns
the difference from the current time so that it can be put into condition statements instead of having to get the
current time and then get the difference.
diff --git a/src/session_p.h b/src/session_p.h
index 182a9d8..916e235 100644
--- a/src/session_p.h
+++ b/src/session_p.h
@@ -520,13 +520,33 @@
 
 NC_MSG_TYPE nc_send_msg_io(struct nc_session *session, int io_timeout, struct lyd_node *op);
 
-int nc_gettimespec_mono(struct timespec *ts);
+/**
+ * @brief Get the current real time if available and optionally add some time to it.
+ *
+ * @param[in,out] ts Timespec structure holding time.
+ * @param[in] msec Time in milliseconds to be added.
+ * @return 0 on success;
+ * @return -1 on error.
+ */
+int nc_gettimespec_real_add(struct timespec *ts, uint32_t msec);
 
-int nc_gettimespec_real(struct timespec *ts);
+/**
+ * @brief Get the current monotonic time if available and optionally add some time to it.
+ *
+ * @param[in,out] ts Timespec structure holding time.
+ * @param[in] msec Time in milliseconds to be added.
+ * @return 0 on success;
+ * @return -1 on error.
+ */
+int nc_gettimespec_mono_add(struct timespec *ts, uint32_t msec);
 
-int32_t nc_difftimespec(const struct timespec *ts1, const struct timespec *ts2);
-
-void nc_addtimespec(struct timespec *ts, uint32_t msec);
+/**
+ * @brief Get time difference based on the current time.
+ *
+ * @param[in] ts Timespec structure holding time from which the current time is going to be subtracted.
+ * @return Time difference in milliseconds.
+ */
+int32_t nc_difftimespec_cur(const struct timespec *ts);
 
 const char *nc_keytype2str(NC_SSH_KEY_TYPE type);