A colon can't be used to separate paths on Windows since they are used
in drive sepecification. Define a macro that can be used as a path
separator string. On Windows, this is defined as ";". It is a ":"
everywhere else.
HAVE_WINDNS is defined when we want to use DNSAPI on Windows which
handles most of the resolver work for us.
NO_LIMIT_FD_SETSIZE is defined if the value of a socket FD does not
imply anything about the number of sockets so far assigned.
I.e. checking if the socket is larger than FD_SETSIZE is incorrect.
