Convert most manual accesses to filedesc->fd_files[] into the appropriate

Convert most manual accesses to filedesc->fd_files[] into the appropriate
holdfp() call.  Fix a number of places where ops were being executed
on the file pointer without holding a private reference to it (mainly
fo_ioctl(), revoke(), and lseek()).

Create procedures in kern_descrip.c to set and clear descriptor flags
and to handle the bootstrap filedesc for proc0.  Replace manual code
elsewhere with calls to the new procedures.

Move getvnode() to kern_descrip.c.  Remove nsmb_getfp().  Use holdfp()
instead.