[Enhance] - systemtap dropwatch.stp consider to add drop reason

[Frank Ch. Eigler]

"xili@redhat.com" <xili@redhat.com> writes:

> [...]

I tweaked your proposed code just a bit and pushed it to git/master.
Thanks!  With future feature sourceware.org/PR33516, it should look even
better (easily decode the enums).

- FChE