boot - Fix two bugs in the ufs code.

boot - Fix two bugs in the ufs code.

* Fix an incorrect start f_seekp position on open().  When open()ing a path
  the fp is used for the directory search.  A seek position left over from
  the directory search can wind up being left in place after the file has
  been located and openned, causing the starting seek position to not be 0.

* Fixes problems with the boot loader not detecting valid modules or a
  valid kernel image.

* Handle an overflow case in fp->f_nindir[] initialization.  This case will
  not normally occur.