Move determination of socket buffer sizes from startup to the first time a
socket is used. The previous code structure assumed that AF_INET sockets
were always available, which is an invalid assumption on IPv6-only systems.

This merges the fololowing revisions from NetBSD:
src/usr.bin/ftp/main.c 1.120
src/usr.bin/ftp/util.c 1.156

PR: bin/162661
Tested by: bz
Obtained from: NetBSD
MFC after: 1 week