pppd: Fix compilation with older glibc or kernel headers (#248)

glibc versions prior to 2.24 do not define SOL_NETLINK and linux kernel
versions prior to 4.3 do not define NETLINK_CAP_ACK. So add fallback
definitions for these macros into pppd/sys-linux.c file.

Also extend description why we call SOL_NETLINK/NETLINK_CAP_ACK option.

Signed-off-by: Pali Rohár <pali@kernel.org>