Interface cleanup: Symlink handling & docs

This removes the ability to start nvim via the following aliases in
favor of just using their command line arguments:

ex        vim -e
exim      vim -E
view      vim -R
gvim      vim -g
gex       vim -eg
gview     vim -Rg
rvim      vim -Z
rview     vim -RZ
rgvim     vim -gZ
rgview    vim -RgZ

This also removes Vi mode (-v) for consistency. From ':help -v':

    -v  Start Ex in Vi mode.  Only makes a difference when the
    executable is called "ex" or "gvim".  For gvim the GUI is not
    started if possible.

src/nvim/buffer.c

@@ -213,7 +213,7 @@ open_buffer (
    * So the modelines have priority over auto commands.
    */
   /* When reading stdin, the buffer contents always needs writing, so set
-   * the changed flag.  Unless in readonly mode: "ls | gview -".
+   * the changed flag.  Unless in readonly mode: "ls | nvim -R -".
    * When interrupted and 'cpoptions' contains 'i' set changed flag. */
   if ((got_int && vim_strchr(p_cpo, CPO_INTMOD) != NULL)
       || modified_was_set               /* ":set modified" used in autocmd */