trap/interrupt/syscall entry from userspace.
Remove special-case "by hand" validation of fs/gs register values as
well as special handling of them in various signal handling paths.
Now, like %ds and %es, they are validated by the hardware on return to
userland.
This paves the way for the use of %fs for per-cpu data on
multiprocessor systems, and fixes an otherwise difficult-to-fix
interaction between threads/clone(2) and USER_LDT.
Discussed in advance with Frank van der Linden.
