Don't enable GCOV by default, make test is not ready for that.

When make test is run with gcov, coverage output files are
created. On the second run, when they are already there,
the server fails to start.
Merge branch 'master' into lua-sockets
lua-sockets: review fixes

Rename evio_is_connected to evio_is_active -- a socket
doesn't have to be connected to be usable with evio.
Use evio_is_active instead of s->init in box_socket/lua_io