build: Package sysctl configuration to raise UDP buffer size on Linux (#7417)

* Provide a sysctl config to raise max UDP buffer size

* Add sysctl config to deb

* Check if `deb-systemd-invoke` is available

Co-authored-by: otbutz <tbutz@optitool.de>

etc/linux-sysctl/30-syncthing.conf

@@ -0,0 +1,3 @@
+# Increase maximum receive socket buffer size to 2MiB for QUIC connections
+# see https://github.com/lucas-clemente/quic-go/wiki/UDP-Receive-Buffer-Size
+net.core.rmem_max = 2097152

etc/linux-sysctl/README.md

@@ -0,0 +1,21 @@
+sysctl configuration to raise UDP buffer size
+===================
+Installation
+-----------
+**Please note:** When you installed syncthing using the official deb package, you can skip the copying.
+
+Copy the file `30-syncthing.conf` to `/etc/sysctl.d/` (root permissions required).
+
+In a terminal run
+```
+sudo sysctl -q --system
+```
+to apply the sysctl changes.
+
+
+Verification
+----------
+You can verify that the new limit is active using
+```
+sysctl net.core.rmem_max
+```