Avoid a dangerous race.

To avoid having a ptrace(2)d process from doing an exec of a suid binary
we strip off the suid bits from the executable if the process is traced.
Later we P_SUGID{,EXEC} in the process flags that stops further ptrace
attempts.

The problem is that between the stripping of suid bits and setting
P_SUGID{,EXEC} there is a pretty large window where the process can sleep
and other processes can attach to it with ptrace.

This is a quick fix for that. We set P_SUGID{,EXEC} early and take care to
remove them (we don't want the process to become tainted unnecessarily) if
exec fails.
XXX - exec needs a lot more work, but people are asking for a solution now.

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