Change the default MDA binary to "maildrop"

The current default is "procmail" for historical reasons, but the
program has been unmaintained for years and its use is not generally
advisable.

This patch changes the default MDA binary to "maildrop", which is a more
modern and reasonable MDA to use.

We keep the courier.Procmail name for now, as that's internal, but it
may be changed later.  Its documentation is updated to note that the
name is just for reference but it actually works with almost any binary.

internal/courier/procmail.go

@@ -18,7 +18,11 @@ var (
 	errTimeout = fmt.Errorf("Operation timed out")
 )
 
-// Procmail delivers local mail via procmail.
+// Procmail delivers local mail by executing a local binary, like procmail or
+// maildrop.  It is named after procmail just for reference, it works with any
+// binary that:
+//  - Receives the email to deliver via stdin.
+//  - Exits with code EX_TEMPFAIL (75) for transient issues.
 type Procmail struct {
 	Binary  string        // Path to the binary.
 	Args    []string      // Arguments to pass.