libsystemd-networkd: introduce sd-pppoe library

This library negotiates a PPPoE channel. It handles the discovery stage and
leaves the session stage to the kernel. A further PPP library is needed to
actually set up a PPP unit (negotatie LCP, IPCP and do authentication), so in
isolation this is not yet very useful.

The test program has two modes:

  # ./test-pppoe

will create a veth tunnel in a new network namespace, start pppoe-server on one
end and this client library on the other. The pppd server will time out as no
LCP is performed, and the client will then shut down gracefully.

  # ./test-pppoe eth0

will run the client on eth0 (or any other netdev), and requires a PPPoE server
to be reachable on the local link.

.gitignore

@@ -168,6 +168,7 @@
 /test-dhcp-option
 /test-dhcp-server
 /test-dhcp6-client
+/test-pppoe
 /test-dns-domain
 /test-icmp6-rs
 /test-ellipsize

Makefile.am

@@ -2958,6 +2958,7 @@ libsystemd_network_la_SOURCES = \
 	src/systemd/sd-icmp6-nd.h \
 	src/systemd/sd-dhcp6-client.h \
 	src/systemd/sd-dhcp6-lease.h \
+	src/systemd/sd-pppoe.h \
 	src/libsystemd-network/sd-dhcp-client.c \
 	src/libsystemd-network/sd-dhcp-server.c \
 	src/libsystemd-network/dhcp-network.c \
@@ -2972,6 +2973,7 @@ libsystemd_network_la_SOURCES = \
 	src/libsystemd-network/ipv4ll-network.c \
 	src/libsystemd-network/ipv4ll-packet.c \
 	src/libsystemd-network/ipv4ll-internal.h \
+	src/libsystemd-network/sd-pppoe.c \
 	src/libsystemd-network/network-internal.c \
 	src/libsystemd-network/network-internal.h \
 	src/libsystemd-network/sd-icmp6-nd.c \
@@ -3031,6 +3033,14 @@ test_ipv4ll_LDADD = \
 	libsystemd-internal.la \
 	libsystemd-shared.la
 
+test_pppoe_SOURCES = \
+	src/systemd/sd-pppoe.h \
+	src/libsystemd-network/test-pppoe.c
+
+test_pppoe_LDADD = \
+	libsystemd-network.la \
+	libsystemd-shared.la
+
 test_icmp6_rs_SOURCES = \
 	src/systemd/sd-dhcp6-client.h \
 	src/systemd/sd-icmp6-nd.h \
@@ -3060,6 +3070,9 @@ tests += \
 	test-icmp6-rs \
 	test-dhcp6-client
 
+manual_tests += \
+	test-pppoe
+
 # ------------------------------------------------------------------------------
 if ENABLE_TERMINAL
 noinst_LTLIBRARIES += \