change errno.h to cerrno

The former is deprecated by C++14. It's also functionally the same.

From the standard:

19.4
The header<cerrno>is described in Table 43. Its contents are the same as
the POSIX header<errno.h>,except that errno shall be defined as a macro.
[Note: The intent is to remain in close alignment with the POSIX
standard.] A separate errno value shall be provided for each thread.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/net/Resolver.cxx

@@ -44,9 +44,10 @@
 #include <net/if.h>
 #endif
 
+#include <cerrno>
+
 #include <string.h>
 #include <stdio.h>
-#include <errno.h>
 
 static inline bool
 ai_is_passive(const struct addrinfo *ai)

src/net/SocketDescriptor.cxx

@@ -42,7 +42,8 @@
 #include <netinet/tcp.h>
 #endif
 
-#include <errno.h>
+#include <cerrno>
+
 #include <string.h>
 
 int

src/net/SocketError.hxx

@@ -27,7 +27,7 @@
 #include <winsock2.h>
 typedef DWORD socket_error_t;
 #else
-#include <errno.h>
+#include <cerrno>
 typedef int socket_error_t;
 #endif