eval.c: Rename has('neovim') feature to has('nvim')

To follow the convention of 'nvim' for technical references

src/nvim/eval.c

@@ -9842,7 +9842,7 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     "windows",
     "winaltkeys",
     "writebackup",
-    "neovim",
+    "nvim",
     NULL
   };
 

src/nvim/testdir/test86.in

@@ -9,7 +9,7 @@ STARTTEST
 :so small.vim
 :set encoding=latin1
 :set noswapfile
-:if !has('python') || has('neovim') | e! test.ok | wq! test.out | endif
+:if !has('python') || has('nvim') | e! test.ok | wq! test.out | endif
 :lang C
 :fun Test()
 :py import vim