portlist: report a better process name for .Net on linux.

Fixes #1440.

Signed-off-by: David Anderson <danderson@tailscale.com>

portlist/clean_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package portlist
+
+import "testing"
+
+func TestArgvSubject(t *testing.T) {
+	tests := []struct {
+		in   []string
+		want string
+	}{
+		{
+			in:   nil,
+			want: "",
+		},
+		{
+			in:   []string{"/usr/bin/sshd"},
+			want: "sshd",
+		},
+		{
+			in:   []string{"/bin/mono"},
+			want: "mono",
+		},
+		{
+			in:   []string{"/nix/store/x2cw2xjw98zdysf56bdlfzsr7cyxv0jf-mono-5.20.1.27/bin/mono", "/bin/exampleProgram.exe"},
+			want: "exampleProgram",
+		},
+		{
+			in:   []string{"/bin/mono", "/sbin/exampleProgram.bin"},
+			want: "exampleProgram.bin",
+		},
+	}
+
+	for _, test := range tests {
+		got := argvSubject(test.in...)
+		if got != test.want {
+			t.Errorf("argvSubject(%v) = %q, want %q", test.in, got, test.want)
+		}
+	}
+}