Tidy up fd-related functions called by do_execve

o assert in each one that fdp is not shared
o remove unnecessary NULL checks - all userspace processes have fdtables
and kernel processes cannot execve
o remove comments about the danger of fd_ofiles getting reallocated - fdtable
is not shared and fd_ofiles could be only reallocated if new fd was about to be
added, but if that was possible the code would already be buggy as setugidsafety
work could be undone

MFC after:	1 week