wks: Take name of sendmail from configure.

* configure.ac (NAME_OF_SENDMAIL): New ac_define.
* tools/send-mail.c (run_sendmail): Use it.

We used to ac_subst the SENDMAIL in the old keyserver via mail script.
We can reuse this to avoid a fixed name for sendmail in the
send-mail.c helper.

(cherry picked from commit 08147f8bbdca40c98c2a094fa48fab15b8339c80)

• Signed-off-by: Werner Koch <wk@gnupg.org>
• GnuPG-bug-id: T4886