change-(with-)nth: apply value after current one

- Find current value in bound list and apply next valid,
  non-current one
- Keep rotating bound list as a position hint, so repeated
  values in the list still reach every position
- Treat empty --nth and '..' as the same value
- Ignore invalid nth values, also in transform-nth, as with-nth
  does. Previously, they restored default --nth
- Previously, first value of rotated list was applied blindly
    - Key press spent on value already applied, e.g. first value
      equal to initial --nth
    - Stuck on change-nth printed by transform (new action on
      each run), as it always applied first value
- Add nextValue helper for reuse by change-layout

man/man1/fzf.1

@@ -2060,8 +2060,8 @@ A key or an event can be bound to one or more of the following actions.
     \fBchange\-list\-label(...)\fR       (change \fB\-\-list\-label\fR to the given string)
     \fBchange\-multi\fR                 (enable multi-select mode with no limit)
     \fBchange\-multi(...)\fR            (enable multi-select mode with a limit or disable it with 0)
-    \fBchange\-nth(...)\fR              (change \fB\-\-nth\fR option; rotate through the multiple options separated by '|')
-    \fBchange\-with\-nth(...)\fR         (change \fB\-\-with\-nth\fR option; rotate through the multiple options separated by '|')
+    \fBchange\-nth(...)\fR              (change \fB\-\-nth\fR option; rotate through the multiple options separated by '|', starting from the one after the current option)
+    \fBchange\-with\-nth(...)\fR         (change \fB\-\-with\-nth\fR option; rotate through the multiple options separated by '|', starting from the one after the current option)
     \fBchange\-pointer(...)\fR          (change \fB\-\-pointer\fR option)
     \fBchange\-preview(...)\fR          (change \fB\-\-preview\fR option)
     \fBchange\-preview\-label(...)\fR    (change \fB\-\-preview\-label\fR to the given string)