net/if.h: add some missing IFF_ constants
add getifaddrs

supports ipv4 and ipv6, but not the "extended" usage where
usage statistics and other info are assigned to ifa_data members
of duplicate entries with AF_PACKET family.
include/ifaddrs.h: add prototypes for get/freeifaddrs
getifaddrs: less malloc
getifaddrs: one less indent level
getifaddrs: use if_nameindex to enumerate interfaces
getifaddrs: remove unused label
fix type error in pthread_create, introduced with pthread_getattr_np