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

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.