fixup_mmaps_after_fork failure with cygwin-3.0.0

[Ken Brown <kbrown at cornell dot edu>]

I'm getting a lot of errors like this when building emacs from its git repo:

EMACSLOADPATH= '../src/emacs.exe' -batch --no-site-file --no-site-lisp --eval 
'(setq load-prefer-newer t)'  -f batch-byte-compile ../../master/lisp/finder.el
       1 [main] emacs 26848 fixup_mmaps_after_fork: VirtualProtectEx failed for 
MAP_PRIVATE address 0x6FFFFCD0000, Win32 error 5
     190 [main] emacs 26848 C:\Users\kbrown\src\emacs\x86_64\src\emacs.exe: *** 
fatal error in forked process - recreate_mmaps_after_fork_failed
     619 [main] emacs 26848 cygwin_exception::open_stackdumpfile: Dumping stack 
trace to emacs.exe.stackdump
       1 [main] emacs 27052 fork: child -1 - forked process 26848 died 
unexpectedly, retry 0, exit code 0x100, errno 11


A bisection points to the following as the first bad commit:

commit 69cc7a068656b5c6ef07ca079a213f801e02e650
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Sun Jan 27 13:15:15 2019 +0100

     Cygwin: fork: restrict parent handle perms and drop handle after use


I'll try to do some debugging and/or provide further information tomorrow, but I 
thought I'd start with this preliminary report.

Ken

P.S. I meant to try this during the testing phase prior to the release of 3.0.0, 
but I didn't get to it until today.  Sorry.

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