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
```

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 = ''