To support in-cygwin package managers, the fork() implementation must
not rely on .exe and .dll files to stay in their original location, as
the package manager's job is to replace these files. Instead, we use
the hardlinks to the original binaries in /var/run/cygfork/ to create
the child process during fork, and let the main.exe.local file enable
the "DotLocal Dll Redirection" feature for dlls.
The (probably few) users that need an update-safe fork manually have to
create the /var/run/cygfork/ directory for now, using:
mkdir --mode=a=rwxt /var/run/cygfork