keymap: Do not use vim_isIDc in keymap.c

Note: there are three changes to ascii_isident. Reverting first two (in 
find_special_key and first in get_special_key_code) normally fails the new test 
with empty &isident, but reverting the third does not. Hence adding `>` to 
&isident.

Ref vim/vim#2389.

test/functional/ex_cmds/map_spec.lua

@@ -0,0 +1,21 @@
+local helpers = require("test.functional.helpers")(after_each)
+
+local eq = helpers.eq
+local feed = helpers.feed
+local meths = helpers.meths
+local clear = helpers.clear
+local command = helpers.command
+
+describe(':*map', function()
+  before_each(clear)
+
+  it('are not affected by &isident', function()
+    meths.set_var('counter', 0)
+    command('nnoremap <C-x> :let counter+=1<CR>')
+    meths.set_option('isident', ('%u'):format(('>'):byte()))
+    command('nnoremap <C-y> :let counter+=1<CR>')
+    -- &isident used to disable keycode parsing here as well
+    feed('\24\25<C-x><C-y>')
+    eq(4, meths.get_var('counter'))
+  end)
+end)