util/sysresources, magicsock: scale DERP buffer based on system memory

This adds the util/sysresources package, which currently only contains a
function to return the total memory size of the current system.

Then, we modify magicsock to scale the number of buffered DERP messages
based on the system's available memory, ensuring that we never use a
value lower than the previous constant of 32.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ib763c877de4d0d4ee88869078e7d512f6a3a148d

util/sysresources/memory_bsd.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build freebsd || openbsd || dragonfly || netbsd
+
+package sysresources
+
+import "golang.org/x/sys/unix"
+
+func totalMemoryImpl() uint64 {
+	val, err := unix.SysctlUint64("hw.physmem")
+	if err != nil {
+		return 0
+	}
+	return val
+}