Simplify target_async hook interface

Simplify target_async hook interface

All callers of target_async pass it the same callback
(inferior_event_handler).  Since both common code and target backends
need to be able to put the target in and out of target async mode at
any given time, there's really no way that a different callback could
be passed.  This commit simplifies things, and removes the indirection
altogether.  Bonus: with this, gdb's target_async method ends up with
the same signature as gdbserver's.

Tested on x86_64 Fedora 20, native and gdbserver.

gdb/ChangeLog:
2015-03-25  Pedro Alves  <palves@redhat.com>

	* target.h <to_async>: Replace 'callback' and 'context' parameters
	with boolean 'enable' parameter.
	(target_async): Replace CALLBACK and CONTEXT parameters with
	boolean ENABLE parameter.
	* inf-loop.c (inferior_event_handler): Adjust.
	* linux-nat.c (linux_nat_attach, linux_nat_resume)
	(linux_nat_resume): Adjust.
	(async_client_callback, async_client_context): Delete.
	(handle_target_event): Call inferior_event_handler directly.
	(linux_nat_async): Replace 'callback' and 'context' parameters
	with boolean 'enable' parameter.  Adjust.  Remove references to
	async_client_callback and async_client_context.
	(linux_nat_close): Adjust.
	* record-btrace.c (record_btrace_async): Replace 'callback' and
	'context' parameters with boolean 'enable' parameter.  Adjust.
	(record_btrace_resume): Adjust.
	* record-full.c (record_full_async): Replace 'callback' and
	'context' parameters with boolean 'enable' parameter.  Adjust.
	(record_full_resume, record_full_core_resume): Adjust.
	* remote.c (struct remote_state) <async_client_callback,
	async_client_context>: Delete fields.
	(remote_start_remote, extended_remote_attach_1, remote_resume)
	(extended_remote_create_inferior): Adjust.
	(remote_async_serial_handler): Call inferior_event_handler
	directly.
	(remote_async): Replace 'callback' and 'context' parameters with
	boolean 'enable' parameter.  Adjust.
	* top.c (gdb_readline_wrapper_cleanup, gdb_readline_wrapper):
	Adjust.
	* target-delegates.c: Regenerate.