Add portable is localhost function

This removes the utils_is_ipv4_local() call and move to a generic
portable way of checking if the sockaddr is localhost (both ipv4 and
ipv6 are supported).

Reported-by: Nick Mathewson <nickm@torproject.org>
Signed-off-by: David Goulet <dgoulet@ev0ke.net>
Fix: bad libc detection on system with libcap.so

The binary "yes" is much more generic and has a standard path in BSD,
Linux and OS X. Furthermore, it cannot be linked with libcap.so. But
this patch improves the grep regex to only detect "libc.so".

Fixes #21

Signed-off-by: David Goulet <dgoulet@ev0ke.net>
Fix: README typos

Corrected some mistakes in the README file.

Closes #22

Signed-off-by: David Goulet <dgoulet@ev0ke.net>