Fix SSH_PUBKEYINFO, limit characters, add tests

We fix a bad_bufptr() failure from a previous commit. We now limit
the allowed characters to those that will definitely be safe
in a shell. Some scripts/programs may use arbitrary environment
variables without escaping correctly - that could be a problem
in a restricted environment.

The current allowed set is a-z A-Z 0-9 .,_-+@

This also adds a test for SSH_PUBKEYINFO, by default it only runs
under github actions (or "act -j build").

test/test_svrauth.py

@@ -0,0 +1,30 @@
+from test_dropbear import *
+import signal
+import queue
+import socket
+import os
+from pathlib import Path
+
+# Tests for server side authentication
+
+# Requires keyfile and authorized_keys set up in github action build.yml
+@pytest.mark.skipif('DBTEST_IN_ACTION' not in os.environ, reason="DBTEST_PUBKEYINFO not set")
+def test_pubkeyinfo(request, dropbear):
+	kf = str(Path.home() / ".ssh/id_dropbear_key2")
+	r = dbclient(request, "-i", kf, "echo -n $SSH_PUBKEYINFO", capture_output=True)
+	# stop at first space
+	assert r.stdout.decode() == "key2"
+
+@pytest.mark.skipif('DBTEST_IN_ACTION' not in os.environ, reason="DBTEST_PUBKEYINFO not set")
+def test_pubkeyinfo_special(request, dropbear):
+	kf = str(Path.home() / ".ssh/id_dropbear_key3")
+	r = dbclient(request, "-i", kf, "echo -n $SSH_PUBKEYINFO", capture_output=True)
+	# comment contains special characters so the SSH_PUBKEYINFO should not be set
+	assert r.stdout.decode() == ""
+
+@pytest.mark.skipif('DBTEST_IN_ACTION' not in os.environ, reason="DBTEST_PUBKEYINFO not set")
+def test_pubkeyinfo_okchar(request, dropbear):
+	kf = str(Path.home() / ".ssh/id_dropbear_key4")
+	r = dbclient(request, "-i", kf, "echo -n $SSH_PUBKEYINFO", capture_output=True)
+	# comment contains special characters so the SSH_PUBKEYINFO should not be set
+	assert r.stdout.decode() == "key4,char"