Races galore...  General rule: as soon as it's in descriptor table,
it's over; another thread might have started IO on it/dup2() it
elsewhere/dup2() something *over* it/etc.  fd_install() is the very
last step one should take - it's a point of no return.

Besides, the damn thing leaked on failure exits...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>