Fix compiler warning and signedness issue (#401)

* Fix compiler warning and signedness issue

This commit fixes a compiler warning I was getting with regard to signed
and unsigned types. On line 794 of `Discovery.hh` we use the recvfrom
but we use an unsigned int to check if the error is correct or not. By
default recvfrom returns an `ssize_t` not a `uint16_t`. It seems a fix
for this has landed in garden but has not been backported to citadel or
fortress.

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

* Switch to `int64_t` to make m$s$ w1nd0ws happy

Rant: At this point I'm not sure that a true C++ compiler and stdlib exist. There is
m$s$ C++ thats a different language from GNU C++.

Look at all these plebians using windows...

Signed-off-by: Arjo Chakravarty <arjoc@intrinsic.ai>

include/gz/transport/Discovery.hh

@@ -788,7 +788,7 @@ namespace ignition
         sockaddr_in clntAddr;
         socklen_t addrLen = sizeof(clntAddr);
 
-        uint16_t received = recvfrom(this->sockets.at(0),
+        int64_t received = recvfrom(this->sockets.at(0),
               reinterpret_cast<raw_type *>(rcvStr),
               this->kMaxRcvStr, 0,
               reinterpret_cast<sockaddr *>(&clntAddr),
@@ -820,7 +820,7 @@ namespace ignition
           // unexpected size, then we ignore the message.
 
           // If-condition for version 8+
-          if (len + sizeof(len) == received)
+          if (len + sizeof(len) == static_cast<uint16_t>(received))
           {
             std::string srcAddr = inet_ntoa(clntAddr.sin_addr);
             uint16_t srcPort = ntohs(clntAddr.sin_port);