Add support for shared tables (#1988)

Currently tables can be created in two modes:
1. private to the ebpf program
2. shared among all the ebpf programs created by the same user-space
process

This commit extends bcc allowing to create a table that is shared among
a set of specific ebpf programs.
This is useful when creating network functions that are composed
by more than 1 ebpf program, a map is shared among all the programs of
the network function but isolated from different instances of that
function.

Signed-off-by: Mauricio Vasquez B <mauricio.vasquez@polito.it>