confd: set up compat authorized_keys symlink for netopeer2-server

Apparently netopeer2-server expects public keys for users to be in the
canonical place (~/.ssh/authorized_keys).  Infix use a custom location
in /var/run, maintained by the ietf-system plugin, which OpenSSH knows
but netopeer2-server doesn't.

It seems to be possible to add some sort of "pattern" to netopeer2 to
indicate where to look for public keys.  This is added when building it
but Buildroot does not expose this as a setting.  So for now, this seem
to be the least intrusive way of handling it.  The owner is root, so a
malicious user cannot change the link or contents without some other
form of privilege escalation first.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/confd/src/confd/ietf-system.c

@@ -334,7 +334,7 @@ static int change_clock(sr_session_ctx_t *session, uint32_t sub_id, const char *
 	}
 
 	remove("/etc/localtime+");
-	if (systemf("ln -s /usr/share/zoneinfo/%s /etc/localtime+", timezone)) {
+	if (systemf("ln -sf /usr/share/zoneinfo/%s /etc/localtime+", timezone)) {
 		ERROR("No such timezone %s", timezone);
 		return SR_ERR_VALIDATION_FAILED;
 	}
@@ -636,6 +636,15 @@ static int sys_add_new_user(char *name)
 	}
 	DEBUG("New user \"%s\" created\n", name);
 
+	/*
+	 * OpenSSH in Infix has been set up to use /var/run/sshd/%s.keys
+	 * but libSSH used by netopeer2-server still reads the classic
+	 * /home/%s/.ssh/authorized_keys file.  This creates a both the
+	 * directory and the symlink owned by root to prevent tampering.
+	 */
+	fmkpath(0750, "/home/%s/.ssh", name);
+	systemf("ln -sf /var/run/sshd/%s.keys /home/%s/.ssh/authorized_keys", name, name);
+
 	return SR_ERR_OK;
 }