tracepoint argument handling needs to be updated for newer kernels

[rganesan]

Recent kernels provide easier access to tracepoint char strings, instead of the __data_loc indirection. ply needs to be updated to handle char * fields directly instead of the roundabout dyn mechanism.

[rganesan]

"ply 'tracepoint:syscalls/sys_enter_openat { printf("%v(%v): %s\n", comm, uid, str(data->filename)); }'" works as recommended by ismhong's comment on issue 87 (#87 (comment))