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 cab reuse this to avoid a fixed name for sendmail in the
send-mail.c helper.

• Signed-off-by: Werner Koch <wk@gnupg.org>