fix PTRACE_PEEKUSER, PTRACE_POKEUSER, PTRACE_GETREGS.

support PTRACE_SETREGS.
  In struct process, add 'unsigned long *ptrace_debugreg', instead of 'struct user *userp'.
  debug registers are read/written from/to ptrace_debugreg, save/restore in schedule().
  most general registers are proc->uctx.
  fs_base is proc->thread.tlsblock_base.
  gs_base,ds,es,fs,gs and orig_rax are uncompleted.
  other members in 'struct user' are ignored, same as Linux implementation.

refs #257
refs #373
refs #263

kernel/include/process.h

@@ -352,7 +352,7 @@ struct process {
 	cpu_set_t cpu_set;
 	unsigned long saved_auxv[AUXV_LEN];
 
-	struct user *userp;
+	unsigned long *ptrace_debugreg;	/* debug registers for ptrace */
 };
 
 struct process_vm {