This fixes CONFIG_UID16 problems on x86-64 as discussed earlier.

CONFIG_UID16 now only selects the inclusion of kernel/uid16.c, all
conversions are triggered dynamically based on type sizes.  This allows
x86-64 to both include uid16.c for emulation purposes, but not truncate
uids to 16bit in sys_newstat. 

- Replace the old macros from linux/highuid.h with new SET_UID/SET_GID
  macros that do type checking. Based on Linus' proposal.

- Fix everybody to use them.

- Clean up some cruft in the x86-64 32bit emulation allowed by this
  (other 32bit emulations could be cleaned too, but I'm too lazy for 
  that right now) 

- Add one missing EOVERFLOW check in x86-64 32bit sys_newstat while
  I was at it.