Extend faq.using to discuss fork failures

[Christopher Faylor]

On Fri, Aug 19, 2011 at 12:06:57PM -0400, Charles Wilson wrote:
>As stated earlier, recommending the use of msvcrt spawn*() is very
>un-cygwin -- even if it were possible to do so (see above wrt
>/usr/i686-pc-mingw32/sys-root/mingw/*).  Simply using *cygwin's*
>(deprecated) spawn*() functions won't help much either unless the user
>REALLY knows what they are doing; and if THAT were the case, (a) they
>wouldn't need this FAQ, and (b) they'd know to use the POSIX *_spawn()
>functions instead.

Bingo.  Right.

Those are even better reasons for not including the description than
"It's un-Cygwin".  The implications of converting from the use of
fork()/exec() to spawn requires expertise so casually mentioning this as
a "solution" is bound to generate "How exactly do I do that?  Do I just
replace all calls to fork() with spawn()?" type questions.

cgf