getdigits: introduce strict, def parameters

Problem:
During a refactor long ago, we changed the `getdigits_*` familiy of
functions to abort on overflow.  But this is often wrong, because many
of these codepaths are handling user input.

Solution:
Decide at each call-site whether to use "strict" mode.

fix #5555

test/functional/ex_cmds/excmd_spec.lua

@@ -0,0 +1,32 @@
+local helpers = require("test.functional.helpers")(after_each)
+local command = helpers.command
+local eq = helpers.eq
+local clear = helpers.clear
+local pcall_err = helpers.pcall_err
+local assert_alive = helpers.assert_alive
+
+describe('Ex cmds', function()
+  before_each(function()
+    clear()
+  end)
+
+  it('handle integer overflow from user-input #5555', function()
+    command(':9999999999999999999999999999999999999999')
+    command(':later 9999999999999999999999999999999999999999')
+    command(':echo expand("#<9999999999999999999999999999999999999999")')
+    command(':lockvar 9999999999999999999999999999999999999999')
+    command(':winsize 9999999999999999999999999999999999999999 9999999999999999999999999999999999999999')
+    eq('Vim(tabnext):E474: Invalid argument',
+      pcall_err(command, ':tabnext 9999999999999999999999999999999999999999'))
+    eq('Vim(Next):E939: Positive count required',
+      pcall_err(command, ':N 9999999999999999999999999999999999999999'))
+    eq('Vim(menu):E329: No menu "9999999999999999999999999999999999999999"',
+      pcall_err(command, ':menu 9999999999999999999999999999999999999999'))
+    eq('Vim(bdelete):E939: Positive count required',
+      pcall_err(command, ':bdelete 9999999999999999999999999999999999999999'))
+    eq('Vim(retab):E487: Argument must be positive',
+      pcall_err(command, ':retab 9999999999999999999999999999999999999999'))
+    assert_alive()
+  end)
+end)
+