cleaned up ancient initialization code in lib/cygwin_crt0.c

[Christopher Faylor]

For Cygwin 1.7.x, I've cleaned up the ancient code in the file mentioned
in the subject which attempted to deal with old binaries that supplied
their own "user_data" information rather than using __cygwin_user_data.
That means that I've broken backwards compatibility with binaries that
were created before the millenium.

There are a few other places where we've done that in 1.7.x so I don't
feel bad about this at all but I thought I should document this fact
here regardless.

FYI.
cgf