use symbol annotation for `argname` keyword

Using below ktap one-liner:
ktap -e 's = {}; trace probe:vfs_read,probe:vfs_write { s[argname] += 1 } trace_end { histogram(s) }'

Before:
                          value ------------- Distribution ------------- count
                             kp0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@          71
                             kp1 |@@@@@@@@@                              24

After:

                          value ------------- Distribution ------------- count
                        vfs_read |@@@@@@@@@@@@@@@@@@@@@@                 4
                       vfs_write |@@@@@@@@@@@@@@@@                       3

Users may like those keys to be "vfs_read", "vfs_write", and not "kp0", "kp1".

This issue was reported by Brendan Gregg.

Now the `argname` represent the event_name in debugfs.
kprobe:		/sys/kernel/debug/tracing/ktap_kprobes_$pid/$event_name
kretprobe:	/sys/kernel/debug/tracing/ktap_kprobes_$pid/ret_$event_name
uprobe:		/sys/kernel/debug/tracing/ktap_uprobes_$pid/$event_name
uretprobe:	/sys/kernel/debug/tracing/ktap_uprobes_$pid/ret_$event_name

For example:
./ktap -e 's = {}; trace probe:vfs_read,probe:vfs_write, probe:vfs_read%return, probe:vfs_write%return{ s[argname] += 1 } trace_end { histogram(s) }'

Tracing... Ctrl-C to end.
^C
                          value ------------- Distribution ------------- count
                        vfs_read |@@@@@@@@@@@@@@@@@@                     276
		    ret_vfs_read |@@@@@@@@@@@@@@@@@@                     275
		   ret_vfs_write |                                       14
	               vfs_write |                                       12

Signed-off-by: Jovi Zhangwei <jovi.zhangwei@gmail.com>