ha: Make receive buffer size for the HA socket configurable

If there are lots of SAs to be synced, the default might be too low
and messages and SAs get dropped.  The new default is already 8 MiB,
which should work fine for lots of SAs.  The code mirrors the one in
the kernel-netlink plugin (but with a guard around SO_RCVBUFFORCE, even
though this plugin is mostly used on Linux as well).

conf/plugins/ha.opt

@@ -9,6 +9,17 @@ charon.plugins.ha.buflen = 2048
 	also transmitted so depending on the DH group the HA messages can get quite
 	big (the default should be fine up to _modp4096_).
 
+charon.plugins.ha.receive_buffer_size = 8388608
+	Maximum receive buffer size for the HA socket in bytes.
+
+	Maximum receive buffer size for the HA socket in bytes. This value controls
+	how many bytes of HA messages can be queued to the socket. If set to 0, the
+	default from `/proc/sys/net/core/rmem_default` will apply. Note that the
+	kernel doubles the configured value to account for overhead. To exceed the
+	system-wide maximum from `/proc/sys/net/core/rmem_max`, special privileges
+	(CAP_NET_ADMIN) are necessary, otherwise, the kernel silently caps the
+	value.
+
 charon.plugins.ha.fifo_interface = yes
 
 charon.plugins.ha.heartbeat_delay = 1000