re: uarea related cleanup

[matthew green <mrg%eterna.com.au@localhost>]

   To finish uarea clean-up and remove struct user (and sys/user.h), I would
   like to make few changes in our MD code:
   
   - Rename lwp_t::l_addr to l_pcb, and replace USER_TO_UAREA/UAREA_TO_USER
     et al macros with uvm_lwp_getuarea() and uvm_lwp_setuarea() routines.
   
   - Minimise direct access of lwp_t::l_addr/l_pcb by replacing them with a new
     lwp_getpcb(lwp_t *l) routine.  Mainly assembler would use l_pcb directly.


rmind showed me a small version of this for arch/sparc and i liked it
quite a lot.  i'd be happy to see this.


.mrg.