Change spawn() to be a completion token-based async operation.

Added new spawn() overloads that conform to the requirements for
asynchronous operations. These overloads also support cancellation. When
targeting C++11 and later these functions are implemented in terms of
Boost.Context directly.

The existing overloads have been retained but are deprecated.

example/cpp03/spawn/parallel_grep.cpp

@@ -8,6 +8,7 @@
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
+#include <boost/asio/detached.hpp>
 #include <boost/asio/dispatch.hpp>
 #include <boost/asio/post.hpp>
 #include <boost/asio/spawn.hpp>
@@ -19,6 +20,7 @@
 #include <iostream>
 #include <string>
 
+using boost::asio::detached_t;
 using boost::asio::dispatch;
 using boost::asio::spawn;
 using boost::asio::strand;
@@ -74,8 +76,10 @@ int main(int argc, char* argv[])
     for (int argn = 2; argn < argc; ++argn)
     {
       std::string input_file = argv[argn];
-      spawn(pool, boost::bind(&search_file, search_string,
-            input_file, output_strand, boost::placeholders::_1));
+      spawn(pool,
+          boost::bind(&search_file, search_string,
+            input_file, output_strand, boost::placeholders::_1),
+          detached_t());
     }
 
     // Join the thread pool to wait for all the spawned tasks to complete.