Change API of attach_kprobe to take a name argument

Change API of attach_kprobe to take a name argument

Per feedback on the attach_kprobe api, change up the arguments to remove
the load_func that typically preceeds the call. Instead, move this
inside the attach_kprobe implementation. Also, this makes attach_kprobe
need to be non-static. The same applies to attach_kretprobe.

Old:
fn = b.load_func("hello", BPF.KPROBE)
BPF.attach_kprobe(fn, "sys_clone")

New:
b.attach_kprobe(event="sys_clone", fn_name="hello")

Note that the kwarg style is not required, but I fixed up the current
usages to provide readability.

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>