Create per service debug logging toggles

Enabling the rspamd debug log drowns out everything else and should be
selected explicitly as needed.

The external test does not require it and removing it makes it much
(~40.5%) faster, since it now does not block on terminal output anymore.

Before:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
  Time (mean ± σ):     151.737 s ±  1.074 s    [User: 0.310 s, System: 0.289 s]
  Range (min … max):   150.321 s … 153.512 s    10 runs
```

After:
```
Benchmark 1: nix build .#hydraJobs.x86_64-linux.external-unstable --rebuild
  Time (mean ± σ):     90.531 s ±  0.557 s    [User: 0.054 s, System: 0.045 s]
  Range (min … max):   89.579 s … 91.278 s    10 runs
```

default.nix

@@ -1026,16 +1026,36 @@ in
       };
     };
 
-    debug = mkOption {
+    debug = {
+      all = mkOption {
         type = types.bool;
         default = false;
         description = ''
-        Whether to enable verbose logging for mailserver related services. This
+          Whether to enable verbose logging for all mailserver related services.
-        intended be used for development purposes only, you probably don't want
+          This intended be used for development purposes only, you probably
-        to enable this unless you're hacking on nixos-mailserver.
+          don't want to enable this unless you're hacking on nixos-mailserver.
         '';
       };
 
+      dovecot = mkOption {
+        type = types.bool;
+        default = cfg.debug.all;
+        defaultText = lib.literalExpression "config.mailserver.debug.all";
+        description = ''
+          Whether to enable verbose logging for Dovecot.
+        '';
+      };
+
+      rspamd = mkOption {
+        type = types.bool;
+        default = cfg.debug.all;
+        defaultText = lib.literalExpression "config.mailserver.debug.all";
+        description = ''
+          Whether to enable verbose logging for Rspamd.
+        '';
+      };
+    };
+
     maxConnectionsPerUser = mkOption {
       type = types.int;
       default = 100;

mail-server/dovecot.nix

@@ -262,7 +262,7 @@ in
 
       extraConfig = ''
         #Extra Config
-        ${lib.optionalString cfg.debug ''
+        ${lib.optionalString cfg.debug.dovecot ''
           mail_debug = yes
           auth_debug = yes
           verbose_ssl = yes

mail-server/rspamd.nix

@@ -67,7 +67,7 @@ in
 
     services.rspamd = {
       enable = true;
-      inherit (cfg) debug;
+      debug = cfg.debug.rspamd;
       locals = {
         "milter_headers.conf" = {
           text = ''

tests/external.nix

@@ -39,7 +39,7 @@
 
         mailserver = {
           enable = true;
-          debug = true;
+          debug.dovecot = true; # enabled for sieve script logging
           fqdn = "mail.example.com";
           domains = [
             "example.com"