Revert stuff related to childfdmap

Revert "os/exec: less broken implementation of fd passing"
    This reverts commit 890bd2bee7f1dc26307a8167121bdafb3f87dcc6.

Revert "Fix the constant that defines the DUP_FDS_TO system call."
    This reverts commit f7581d5820c81ef17f1241e81649825fb2f441af.

Revert "Add support for childfdmap."
    This reverts commit cea46024744a06b11114802d1e467ee0375d7d3f.