net: Add INET_ADDRSTRLEN POSIX define

POSIX defines INET_ADDRSTRLEN and INET6_ADDRSTRLEN as max sizes of
textual form of IP addresses (including terminating NUL):
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/netinet_in.h.html



We already define INET6_ADDRSTRLEN, so it makes sense to define
INET_ADDRSTRLEN too.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>