This adds a similar set of functions to kprobes for userspace probes.
The calling convention is different, however, since the user must
provide a library/binary name and function symbol or address. Add two
simple test cases for both.

I don't see an easy way in this api to add auto-loading support, as in
kprobe functions that start with "kprobe__". Such niceties can come
later.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>