I've just discovered that using the saved sessions menu from Unix

I've just discovered that using the saved sessions menu from Unix
PuTTY causes the child process to inherit a lot of socket fds from
its parent, which is a pain if one of them then ends up holding open
a listening socket which the parent was using for port forwarding
after the parent itself is dead.

Therefore, this checkin sprinkles FD_CLOEXEC throughout the Unix
platform directory wherever there looks like being a long-lived fd.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6917 cda61777-01e9-0310-a592-d414129be87e