Updated squid 2.3 stable 3 for cygwin

Hi,
    I've just joined the development list - I'll be working on cygwin issues
only (possibly more should time permit...). I've attached a diff to update
squid 2.3 stable 3's cygwin support. I have edited configure.in so (and I
quote)
"Ok. Ditch the configure diff .. tell people they need to run autoconf
and autoheader to regenerate the configure info after patching."
I've also created a new file README.cygwin, as I wasn't sure where my notes
on the remaining brokeness should go. Tell me where, and next time I'll do
that.

In the diff I have
* replaced a hard coded constant in dns_internal with MAX_DNS_INCOMING
* put a cygwin specific sys_nerr import in util.c
* added README.cygwin (wasn't sure where the info in there should go)
* changed the platform detect code in config.h.in and configure.in to
recognise the new __CYGWIN__ as well as the deprecated __CYGWIN32__
* in defines.h use the presence of cygwin to set the max_xxx_incoming
defines to 1 rather than 15,15 and 10 (this is a work around for a blocking
io isssue.)

Rob Collins