session UPDATE support signals in poll() calls

Instead of ignoring all the signals, process
them and adjust the timeout accordingly.
diff --git a/src/session_p.h b/src/session_p.h
index 7637d69..17dfe03 100644
--- a/src/session_p.h
+++ b/src/session_p.h
@@ -19,6 +19,7 @@
 
 #define _GNU_SOURCE
 
+#include <poll.h>
 #include <pthread.h>
 #include <stdint.h>
 
@@ -816,6 +817,16 @@
 void nc_realtime_get(struct timespec *ts);
 
 /**
+ * @brief Perform poll(2) with signal support and timeout adjustment.
+ *
+ * @param[in] pfd Poll structure to use.
+ * @param[in] pfd_count Count of FD in @p pfd.
+ * @param[in] timeout_ms Timeout to use.
+ * @return poll(2) return.
+ */
+int nc_poll(struct pollfd *pfd, uint16_t pfd_count, int timeout);
+
+/**
  * @brief Enables/disables TCP keepalives.
  *
  * @param[in] sock Socket to set this option for.