inf-ptrace: Add an event_pipe to be used for async mode in subclasses.

Subclasses of inf_ptrace_target have to opt-in to using the event_pipe
by implementing the can_async_p and async methods.  For subclasses
which do this, inf_ptrace_target provides is_async_p, async_wait_fd
and closes the pipe in the close target method.

inf_ptrace_target also provides wrapper routines around the event pipe
(async_file_open, async_file_close, async_file_flush, and
async_file_mark) for use in target methods such as async.
inf_ptrace_target also exports a static async_file_mark_if_open
function which can be used in SIGCHLD signal handlers.

gdb/inf-ptrace.c

@@ -48,6 +48,9 @@ gdb_ptrace (PTRACE_TYPE_ARG1 request, ptid_t ptid, PTRACE_TYPE_ARG3 addr,
 #endif
 }
 
+/* The event pipe registered as a waitable file in the event loop.  */
+event_pipe inf_ptrace_target::m_event_pipe;
+
 inf_ptrace_target::~inf_ptrace_target ()
 {}
 
@@ -527,3 +530,15 @@ inf_ptrace_target::pid_to_str (ptid_t ptid)
 {
   return normal_pid_to_str (ptid);
 }
+
+/* Implement the "close" target method.  */
+
+void
+inf_ptrace_target::close ()
+{
+  /* Unregister from the event loop.  */
+  if (is_async_p ())
+    async (0);
+
+  inf_child_target::close ();
+}