KWSys SystemTools: Port cygwin path conversion to modern API

The cygwin_conv_to_win32_path function is deprecated in favor of
cygwin_conv_path.  Use the latter.
KWSys Process: Workaround child kill trouble on Cygwin

When we kill a child we send SIGSTOP first so that we can traverse its
child processes recursively.  On unwinding the recursion we then send
SIGKILL.  Current Cygwin has trouble when both signals are sent in quick
succession by the parent process.  Add a usleep(1) after sending the
first signal to give up our time slice and give Cygwin a chance to
process the first signal before sending the second.
Merge topic 'backport-kwsys-cygwin-fixes'

0c7f84ca KWSys Process: Workaround child kill trouble on Cygwin
e604209c KWSys SystemTools: Port cygwin path conversion to modern API