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/globals.h

@@ -601,7 +601,7 @@ EXTERN volatile int full_screen INIT(= FALSE);
  * otherwise only writing some messages */
 
 EXTERN int restricted INIT(= FALSE);
-/* TRUE when started as "rvim" */
+// TRUE when started in restricted mode (-Z)
 EXTERN int secure INIT(= FALSE);
 /* non-zero when only "safe" commands are
  * allowed, e.g. when sourcing .exrc or .vimrc