add valid nick function

src/misc_tools.h

@@ -30,3 +30,9 @@ void alert_window(ToxWindow *self);
 
 /* case-insensitive string compare function for use with qsort - same return logic as strcmp */
 int name_compare(const void *nick1, const void *nick2);
+
+/* Returns true if nick is valid. A valid toxic nick:
+      - cannot be empty
+      - cannot start with a space
+      - must not contain contiguous spaces */
+bool valid_nick(uint8_t *nick);