process-util: add new FORK_CLOEXEC_OFF flag for disabling O_CLOEXEC on remaining fds
Often the fds that shall stay around in the child shall be passed
to a process over execve(), hence add an option to explicitly disable
O_CLOEXEC on them in the child.
(cherry picked from commit 981cfbe046297a18f2cb115ef81202f3bd68d2a3)