gdb: Coalesce/aggregate (async) vCont packets/actions

Currently, with "maint set target-non-stop on", that is, when gdb
connects with the non-stop/asynchronous variant of the remote
protocol, even with "set non-stop off", GDB always sends one vCont
packet per thread resumed.  This patch makes GDB aggregate and
coalesce vCont packets, so we send vCont packets like "vCont;s:p1.1;c"
in non-stop mode too.

Basically, this is done by:

  - Adding a new target method target_commit_resume that is called
    after calling target_resume one or more times.  When resuming a
    batch of threads, we'll only call target_commit_resume once after
    calling target_resume for all threads.

  - Making the remote target defer sending the actual vCont packet to
    target_commit_resume.

Special care must be taken to avoid sending a vCont action with a
"wildcard" thread-id (all threads of process / all threads) when that
would resume threads/processes that should not be resumed.  See
remote_commit_resume comments for details.

Unlike all-stop's remote_resume implementation, this handles the case
of too many actions resulting in a too-big vCont packet, by flushing
the vCont packet and starting a new one.

E.g., imagining that the "c" action in:

  vCont;s:1;c

overflows the packet buffer, we split the actions like:

  vCont;s:1
  vCont;c

Tested on x86_64 Fedora 20, with and without "maint set
target-non-stop on".

Also tested with a hack that makes remote_commit_resume flush the vCont
packet after every action appended (which caught a few bugs).

gdb/ChangeLog:
2016-10-26  Pedro Alves  <palves@redhat.com>

	* inferior.h (ALL_NON_EXITED_INFERIORS): New macro.
	* infrun.c (do_target_resume): Call target_commit_resume.
	(proceed): Defer target_commit_resume while looping over threads,
	resuming them.  Call target_commit_resume at the end.
	* record-btrace.c (record_btrace_commit_resume): New function.
	(init_record_btrace_ops): Install it as to_commit_resume method.
	* record-full.c (record_full_commit_resume): New function.
	(record_full_wait_1): Call the beneath target's to_commit_resume
	method.
	(init_record_full_ops): Install record_full_commit_resume as
	to_commit_resume method.
	* remote.c (struct private_thread_info) <last_resume_step,
	last_resume_sig, vcont_resumed>: New fields.
	(remote_add_thread): Set the new thread's vcont_resumed flag.
	(demand_private_info): Delete.
	(get_private_info_thread, get_private_info_ptid): New functions.
	(remote_update_thread_list): Adjust.
	(process_initial_stop_replies): Clear the thread's vcont_resumed
	flag.
	(remote_resume): If connected in non-stop mode, record the resume
	request and return early.
	(struct private_inferior): New.
	(struct vcont_builder): New.
	(vcont_builder_restart, vcont_builder_flush)
	(vcont_builder_push_action): New functions.
	(MAX_ACTION_SIZE): New macro.
	(remote_commit_resume): New function.
	(thread_pending_fork_status, is_pending_fork_parent_thread): New
	functions.
	(check_pending_event_prevents_wildcard_vcont_callback)
	(check_pending_events_prevent_wildcard_vcont): New functions.
	(process_stop_reply): Adjust.  Clear the thread's vcont_resumed
	flag.
	(init_remote_ops): Install remote_commit_resume.
	* target-delegates.c: Regenerate.
	* target.c (defer_target_commit_resume): New global.
	(target_commit_resume, make_cleanup_defer_target_commit_resume):
	New functions.
	* target.h (struct target_ops) <to_commit_resume>: New field.
	(target_resume): Update comments.
	(target_commit_resume): New declaration.

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