phpfpm service: restructured pool configuration

From @fpletz: Keep poolConfigs option for backwards-compatibility.

The original commit 6b3f5b5a42 was previously
reverted by c7860cae1a but the issues were
resolved.

nixos/modules/services/web-servers/phpfpm/pool-options.nix

@@ -0,0 +1,35 @@
+{ lib }:
+
+with lib; {
+
+  options = {
+
+    listen = mkOption {
+      type = types.str;
+      example = "/path/to/unix/socket";
+      description = ''
+        The address on which to accept FastCGI requests.
+      '';
+    };
+
+    extraConfig = mkOption {
+      type = types.lines;
+      example = ''
+        user = nobody
+        pm = dynamic
+        pm.max_children = 75
+        pm.start_servers = 10
+        pm.min_spare_servers = 5
+        pm.max_spare_servers = 20
+        pm.max_requests = 500
+      '';
+
+      description = ''
+        Extra lines that go into the pool configuration.
+        See the documentation on <literal>php-fpm.conf</literal> for
+        details on configuration directives.
+      '';
+    };
+  };
+}
+