Add support for SCGI systemd socket activation

Add a new command, `network.scgi.open_systemd`, that binds to a file
descriptor passed in via systemd socket activation.

src/rpc/scgi.cc

@@ -2,6 +2,7 @@
 
 #include <algorithm>
 #include <cassert>
+#include <fcntl.h>
 #include <unistd.h>
 #include <sys/un.h>
 #include <torrent/connection_manager.h>
@@ -80,6 +81,20 @@ SCgi::open_named(const std::string& filename) {
   m_path = filename;
 }
 
+void
+SCgi::open_fd(int fd) {
+  torrent::runtime::socket_manager()->open_event_or_throw(this, [&]() {
+      int flags = ::fcntl(fd, F_GETFL, 0);
+      if (flags == -1 || ::fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1)
+        throw torrent::resource_error("Could not set non-blocking on systemd fd: " +
+                                      std::string(std::strerror(errno)));
+      set_file_descriptor(fd);
+      // fd is already bound and listening; no bind()/listen() needed.
+    });
+
+  torrent::connection_manager()->inc_socket_count();
+}
+
 void
 SCgi::open(sockaddr* sa, unsigned int length) {
   try {

src/rpc/scgi.h

@@ -21,6 +21,7 @@ public:
 
   void                open_port(sockaddr* sa, unsigned int length, bool dont_route);
   void                open_named(const std::string& filename);
+  void                open_fd(int fd);
 
   void                activate();