Cygwin: Quash Windows error text to user on fork() error

For a very long time, since 2011 or earlier, fork() has printed an internal
error message when it fails due to a CreateProcess() error.  This patch
quashes the error message as far as the user can tell, but it will still be
present in an strace.

This change is a judgement call based on the fact we now support
RLIMIT_NPROC and so a user limiting the number of subprocesses may hit
more CreateProcess() errors by design.  Don't clutter the scene.

Fixes: 8551087 (* dll_init.c (dll_list::load_after_fork): Don't
clear in_forkee here.)
Signed-off-by: Mark Geisert <mark@maxrnd.com>

Author: mgeisert

winsup/cygwin/fork.cc

@@ -400,6 +400,7 @@ frok::parent (volatile char * volatile stack_here)
 	{
 	  this_errno = geterrno_from_win_error ();
 	  error ("CreateProcessW failed for '%W'", myself->progname);
+	  ch.silentfail (true);
 	  dlls.release_forkables ();
 	  memset (&pi, 0, sizeof (pi));
 	  goto cleanup;