Back out fd optimization introduced in revision 1.280 as it appears to be

really breaking things. Simple "close(0); dup(fd)" does not return descriptor
"0" in some cases. Further, this change also breaks some MAC interactions with
mac_execve_will_transition().  Under certain circumstances, fdcheckstd() can
be called in execve(2) causing an assertion that checks to make sure that
stdin, stdout and stderr reside at indexes 0, 1 and 2 in the process fd table
to fail, resulting in a kernel panic when INVARIANTS is on.

This should also kill the "dup(2) regression on 6.x" show stopper item on the
6.1-RELEASE TODO list.

This is a RELENG_6 candidate.

PR:		kern/87208
Silence from:	des
MFC after:	1 week

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