Cygwin: fix process parent/child relationship after execve

Commit 5a0f2c00aa "Cygwin: fork/exec: fix child process permissions"
removed the PROCESS_DUP_HANDLE handle permission of the parent process
handle in the child to avoid a security problem.

It turned out that this broke the following scenario: If a process forks
and then the parent execs, the child loses the ability to register the
parent's death.  To wit, after the parent died the child process does
not set its own PPID to 1 anymore.

The current exec mechanism copies required handle values (handles to
keep contact to the child processes) into the child_info for the
about-to-be-exec'ed process.  The exec'ed process is supposed to
duplicate these handles.  This fails, given that we don't allow the
exec'ed process PROCESS_DUP_HANDLE access to the exec'ing process since
commit 5a0f2c00aa.

The fix is to avoid the DuplicateHandle calls in the exec'ed process.

This patch sets the affected handles to "inheritable" in the exec'ing
process at exec time.  The exec'ed process just copies the handle values
and resets handle inheritance to "non-inheritable".  The exec'ing
process doesn't have to reset handle inheritance, it exits after setting
up the exec'ed process anyway.

Testcase: $ ssh-agent /bin/sleep 3

ssh-agent forks and the parent exec's sleep.  After sleep exits, `ps'
should show ssh-agent to have PPID 1, and eventually ssh-agent exits.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

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