Constify unpack_varlen_hex & fix fallout

Constify unpack_varlen_hex & fix fallout

I ran into non-const unpack_varlen_hex while working on something
else, and decided to just fix it first.  Ends up constifying a good
deal of remote packet parsing.

gdb/ChangeLog:
2017-09-29  Pedro Alves  <palves@redhat.com>

	* common/rsp-low.c (unpack_varlen_hex): Constify.
	* common/rsp-low.h (unpack_varlen_hex): Constify.
	* linux-nat.c (linux_child_static_tracepoint_markers_by_strid):
	Constify.
	* remote.c (remote_set_permissions, read_ptid)
	(remote_current_thread, remote_get_threads_with_qthreadinfo)
	(remote_static_tracepoint_marker_at)
	(remote_static_tracepoint_markers_by_strid)
	(stop_reply_extract_thread, remote_parse_stop_reply): Constify.
	* tracepoint.c (parse_trace_status, parse_tracepoint_status)
	(parse_tracepoint_definition, parse_tsv_definition)
	(parse_static_tracepoint_marker_definition): Constify.
	* tracepoint.h (parse_static_tracepoint_marker_definition)
	(parse_trace_status, parse_tracepoint_status)
	(parse_tracepoint_definition, parse_tsv_definition): Constify.

gdb/gdbserver/ChangeLog:
2017-09-29  Pedro Alves  <palves@redhat.com>

	* ax.c (gdb_parse_agent_expr): Constify.
	* ax.h (gdb_parse_agent_expr): Constify.
	* mem-break.c (add_breakpoint_condition, add_breakpoint_commands):
	Constify.
	* mem-break.h (add_breakpoint_condition, add_breakpoint_commands): Constify.
	* remote-utils.c (hex_or_minus_one, read_ptid): Constify.
	* remote-utils.h (read_ptid): Constify.
	* server.c (handle_qxfer_exec_file, handle_query, handle_v_cont)
	(process_point_options, process_serial_event): Constify.
	* tracepoint.c (add_tracepoint_action, cmd_qtdp, cmd_qtdpsrc)
	(cmd_qtdv, cmd_qtenable_disable, cmd_qtro, cmd_qtframe, cmd_qtp)
	(cmd_qtbuffer): Constify.