Disallow '.' at the start of a menu item name.
This is the menu path separator character and should be escaped with a
'\' in this case.
Partially fixes#14230. "popup" is still incorrectly matched as the Ex
command.
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
