poller.c poller.h: replace evutil_socket_t with long long

This diff replaces evutil_socket_t with long long, because long
long simplifies the interaction with SWIG.

As explained in the newly-added comment, long long shall be wide
enough to hold the content of an evutil_socket_t.
poller.h: remove unused definition
Add log.c and teach Makefile how to compile it
Add utils.c and teach Makefile how to compile it
Makefile: add rules to build and install the shared library
libneubot: sched(): hide that we are using libevent

In doing that, also fix the implementation of sched(), which
ignored the value of the delta parameter.
libneubot: pass to pollable (and use) an opaque object
libneubot/log.c: print newline at the end of log message
utils.c utils.h: add neubot_listen()
Add strtonum.c and teach Makefile how to compile it
utils.c: fix paramter name

The parameter asks to use IPv6, it does not imply a vague
preference for IPv6 over IPv4.
utils.c: remember to convert port to network byte order
poller: No need to pass the pollable object in the callbacks
Add echo.c echo server and teach Makefile how to compile it