utils.h: remove long-long-related check

We don't represent sockets using long long anymore, so this
check can be removed from utils..
common/utils.h: better checks for invalid sockets

After reading again event2/util.h and winsock documentation, I think
that the following facts are now clear to me:

1) libevent uses a signed socket type on Windows for convenience, since
   the libevent code uses -1 to indicate an invalid socket;

2) Windows uses an unsigned socket type and uses (~0) as the sentinel
   value to indicate that a socket is not valid;

3) therefore, in libight under Unix a socket is valid if it is greater
   than -1 and not greater than UINT_MAX;

4) in libight under Windows a socket (signed because we are based on
   libevent) is valid if it is different from -1 (signed but equal to
   the ~0 value internally used by Windows).

So, modify common/utils.h accordingly and, while there, also implement
an inline function to normalize an invalid socket value (i.e., to be sure
that an invalid socket is always represented by -1 on Unix).