fix tests on FreeBSD
238b761 introduced a test for posix behaviour, but on FreeBSD some
of the structs and constants used aren't defined in <arpa/inet.h>.
Include the appropriate headers to get the tests working again on
FreeBSD.
Fix p_inet_pton on windows
p_inet_pton on Windows should set errno properly for callers.
Rewrite p_inet_pton to handle error cases correctly and add
test cases to exercise this function.