shell: Use job_write_cb for closing stdin

Commit @45525853d352 removed usage of the `job_write_cb` for closing stdin due
to a memory error, but that doesn't work anymore because `job_close_in` closes
stdin immediately, possibly trimming input data before it is fully written.

Since most memory issues with jobs have been fixed, re-add the `job_write_cb`
call to ensure stdin is only closed when it should. Also add tests for scenarios
where using the callback makes a difference.

test/functional/shell/viml_system_spec.lua

@@ -3,8 +3,8 @@
 -- - `systemlist()`
 
 local helpers = require('test.functional.helpers')
-local eq, clear, eval, feed =
-  helpers.eq, helpers.clear, helpers.eval, helpers.feed
+local eq, clear, eval, feed, nvim =
+  helpers.eq, helpers.clear, helpers.eval, helpers.feed, helpers.nvim
 
 
 local function create_file_with_nuls(name)
@@ -55,6 +55,21 @@ describe('system()', function()
     end)
   end)
 
+  describe('passing a lot of input', function()
+    it('returns the program output', function()
+      local input = {}
+      -- write more than 1mb of data, which should be enough to overcome
+      -- the os buffer limit and force multiple event loop iterations to write
+      -- everything
+      for i = 1, 0xffff do
+        input[#input + 1] = '01234567890ABCDEFabcdef'
+      end
+      input = table.concat(input, '\n')
+      nvim('set_var', 'input', input)
+      eq(input, eval('system("cat -", g:input)'))
+    end)
+  end)
+
   describe('passing number as input', function()
     it('stringifies the input', function()
       eq('1', eval('system("cat", 1)'))
@@ -129,6 +144,17 @@ describe('systemlist()', function()
     end)
   end)
 
+  describe('passing a lot of input', function()
+    it('returns the program output', function()
+      local input = {}
+      for i = 1, 0xffff do
+        input[#input + 1] = '01234567890ABCDEFabcdef'
+      end
+      nvim('set_var', 'input', input)
+      eq(input, eval('systemlist("cat -", g:input)'))
+    end)
+  end)
+
   describe('with output containing NULs', function()
     local fname = 'Xtest'