Replace Boost.Asio with standalone Asio library

We had to add an even hackier hack now to get Boost.Process to
interoperate with Asio's IO contexts. This will be replaced later when
we replace Boost.Process.

src/plugin/utils.cpp

@@ -22,7 +22,6 @@
 #include <boost/dll/runtime_symbol_info.hpp>
 #include <boost/process/io.hpp>
 #include <boost/process/pipe.hpp>
-#include <boost/process/posix.hpp>
 #include <boost/process/search_path.hpp>
 #include <boost/process/system.hpp>
 #include <sstream>
@@ -466,7 +465,7 @@ bool send_notification(const std::string& title,
                            << "\">"
                            << xml_escape(this_library.filename().string())
                            << "</a>";
-        } catch (const boost::system::system_error&) {
+        } catch (const std::system_error&) {
             // I don't think this can fail in the way we're using it, but the
             // last thing we want is our notification informing the user of an
             // exception to trigger another exception