mailutils: use system-sendmail instead of sendmailPath

system-sendmail allows all sendmail's to be auto-detected, including on
non-NixOS systems. This is, to me, a better UX than having to manually
override the sendmailPath argument.

In exchange, it is a breach of retro-compatibility. Given right now I
can't see any uses for sendmailPath other than what is supported by
system-sendmail, I didn't keep it, but it'd be possible to allow
sendmailPath to override the choice of sendmail from system-sendmail.

pkgs/tools/networking/mailutils/default.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, fetchpatch, autoreconfHook, dejagnu, gettext, pkgconfig
 , gdbm, pam, readline, ncurses, gnutls, guile, texinfo, gnum4, sasl, fribidi, nettools
-, python, gss, mysql, sendmailPath ? "/run/wrappers/bin/sendmail" }:
+, python, gss, mysql, system-sendmail }:
 
 stdenv.mkDerivation rec {
   name = "${project}-${version}";
@@ -50,7 +50,7 @@ stdenv.mkDerivation rec {
     "--with-gssapi"
     "--with-gsasl"
     "--with-mysql"
-    "--with-path-sendmail=${sendmailPath}"
+    "--with-path-sendmail=${system-sendmail}/bin/sendmail"
   ];
 
   readmsg-tests = let