Cleanup parser of ptrace syscall

Cleanup parser of ptrace syscall

* process.c (print_user_offset_addr): New function for printing offset
address in the tracee's USER area.
(SYS_FUNC(ptrace)): Assign syscall arguments to local variables with
appropriate types.  Print pid argument as int.  Use switch statement
in the code that handles address argument.  Use print_user_offset_addr
to print address in PTRACE_PEEKUSER and PTRACE_POKEUSER requests.