Add support for enabling and disabling tracepoints while a trace

Add support for enabling and disabling tracepoints while a trace
experiment is still running.

gdb/
	* breakpoint.c (disable_breakpoint): Disable all locations
	associated with a tracepoint on target if a trace experiment is
	running.
	(disable_command): Disable a specific tracepoint location on target if
	a trace	experiment is running.
	(do_enable_breakpoint): Enable all locations associated with a
	tracepoint on target if a trace experiment is running.
	(enable_command) Enable a specific tracepoint location on target if a
	trace experiment is running.
	* target.c (update_current_target): Add INHERIT and de_fault clauses for
	to_supports_enable_disable_tracepoint, to_enable_tracepoint and
	to_disable_tracepoint.
	* target.h: Add declaration of struct bp_location.
	(struct target_ops): Add new functions
	to_supports_enable_disable_tracepoint, to_enable_tracepoint and
	to_disable_tracepoint to target operations.
	(target_supports_enable_disable_tracepoint): New macro.
	(target_enable_tracepoint): New macro.
	(target_disable_tracepoint): New macro.
	* remote.c (struct remote_state): Add new field.
	(remote_enable_disable_tracepoint_feature): New.
	(remote_protocol_features): Add new entry.
	(remote_supports_enable_disable_tracepoint): New.
	(remote_enable_tracepoint): New.
	(remote_disable_tracepoint): New.
	(init_remote_ops): Add remote_enable_tracepoint,
	remote_disable_tracepoint and remote_supports_enable_disable_tracepoint
	to remote operations.
	* tracepoint.c (start_tracing): Allow tracing to start without any
	tracepoints enabled with just a warning if they can be re-enabled
	later.
	* NEWS: Add news item for the new behaviour of the enable and disable
	GDB commands when applied to tracepoints.
	Add news items for the new remote packets QTEnable and QTDisable.

gdb/doc/
	* gdb.texinfo: Document change in the behaviour of the enable and
	disable GDB commands when applied to tracepoints.
	Document the EnableDisableTracepoints remote stub feature.
	Document QTEnable and QTDisable in the list of tracepoint packets.

gdb/gdbserver/
	* server.c (handle_query): Add EnableDisableTracepoints to the list
	of supported features.
	* tracepoint.c (clear_installed_tracepoints): Uninstall	disabled
	tracepoints.
	(cmd_qtenable_disable): New.
	(cmd_qtstart): Install tracepoints even if disabled.
	(handle_tracepoint_general_set): Add call to cmd_qtenable_disable on
	receiving a QTEnable or QTDisable packet.
	(gdb_collect): Skip data collection if fast tracepoint is disabled.
	(ust_marker_to_static_tracepoint): Do not ignore disabled static
	tracepoints.
	(gdb_probe): Skip data collection if static tracepoint is disabled.