unix: modsocket: Implement inet_pton() in preference of inet_aton().

inet_pton supports both ipv4 and ipv6 addresses. Interface is also extensible
for other address families, but underlying libc inet_pton() function isn't
really extensible (e.g., it doesn't return length of binary address, i.e. it's
really hardcoded to AF_INET and AF_INET6). But anyway, on Python side, we could
extend it to support other addresses.