cli: config: fix config file save when the file does not exist

cli: config: fix config file save when the file does not exist

The use of realpath(3) in
commit 58ed67992d0ec1fa505026105218fa449f7980b0
Author: Jani Nikula <jani@nikula.org>
Date:   Sun Apr 7 20:15:03 2013 +0300

    cli: config: do not overwrite symlinks when saving config file

broke config file save when the file does not exist, which results in
'notmuch setup' always failing to create a new config file.

Fix by checking ENOENT from realpath(3).