Add port redirection functionality

The library is not fully functional yet. Currently the error checking is
very limited and must be improved. Additionally a host-port target pair
is always expected. This makes host-only or port-only redirection
impossible.

The good thing is - it works!
Add is_redirectable procedure
Work on a copy of sockaddr

The sockaddr structure passed to connect is a constant. We make a copy
in order to avoid problems when this structure is located in read-only
memory.
Allow preserving original target host or port

When the host or port specified is "original" then it's not changed. The
test suite was extended to cover four different possible use cases.
Update README.md