add TCP_INFO and TCP_MD5SIG socket option related structures
add netinet/igmp.h and multicast groups to netinet/in.h

based on patch by Timo Teräs.
add sys/quota.h and quotactl syscall wrapper

based on patch by Timo Teräs.
implement legacy function herror

based on patch by Timo Teräs; greatly simplified to use fprintf.