There are some problems with zsh's pathname handling that don't work
when cygwin's (poorly thought out) drive letter hacks are in effect:

1) "//<drive-letter>/dir" is a valid path.  builtin.c's fixdir()
   strips off double slashes, which breaks things.

2) If you chdir("..") while in //D you're put in "/" but "/D" is not a
   directory in "/".  This breaks zgetdir().  (there is actually
   another bug in cygwin that breaks zgetdir(), but that is a moot
   point after my fix)

The fixes:

For #1, I just allow a leading // under cygwin.

For #2, I just have zgetdir() call getcwd() under cygwin.  I
considered having zgetdir() use getcwd() for all systems that had it.
I know the Linux kernel now has a getcwd() syscall, which is
presumbably much faster than zgetdir().  But I backed down from this
for fear that all the magic going on in zgetdir() works around bugs in
some system's implementations of getcwd().