rss2email module: init

Also adding `system-sendmail` package for sharing the code with other
modules or packages needing it.

pkgs/servers/mail/system-sendmail/default.nix

@@ -0,0 +1,36 @@
+{ stdenv, writeText }:
+
+let script = writeText "script" ''
+  #!/bin/sh
+
+  if command -v sendmail > /dev/null 2>&1 && [ "$(command -v sendmail)" != "{{MYPATH}}" ]; then
+    exec sendmail "$@"
+  elif [ -x /run/wrappers/bin/sendmail ]; then
+    exec /run/wrappers/bin/sendmail "$@"
+  elif [ -x /run/current-system/sw/bin/sendmail ]; then
+    exec /run/current-system/sw/bin/sendmail "$@"
+  else
+    echo "Unable to find system sendmail." >&2
+    exit 1
+  fi
+''; in
+stdenv.mkDerivation {
+  name = "system-sendmail-1.0";
+
+  src = script;
+
+  phases = [ "buildPhase" ];
+  buildPhase = ''
+    mkdir -p $out/bin
+    < $src sed "s#{{MYPATH}}#$out/bin/sendmail#" > $out/bin/sendmail
+    chmod +x $out/bin/sendmail
+  '';
+
+  meta = with stdenv.lib; {
+    description = ''
+      A sendmail wrapper that calls the system sendmail. Do not install as system-wide sendmail!
+    '';
+    platforms = platforms.unix;
+    maintainers = with maintainers; [ ekleog ];
+  };
+}