Constify some linespec functions

This changes a few linespec functions to work on "const char *" and
then fixes up all the callers.  This allows further constification
elsewhere.

gdb/ChangeLog
2017-09-27  Tom Tromey  <tom@tromey.com>

	* tracepoint.c (info_scope_command): Constify.
	* python/python.c (gdbpy_decode_line): Constify.
	* python/py-breakpoint.c (bppy_init): Constify.
	* mi/mi-cmd-break.c (mi_cmd_break_insert_1): Constify.
	* location.h: (new_linespec_location)
	(string_to_event_location_basic, string_to_event_location):
	Constify.
	* location.c (new_linespec_location)
	(string_to_event_location_basic, string_to_event_location):
	Constify.
	* linespec.h (decode_line_with_current_source)
	(decode_line_with_last_displayed, linespec_lex_to_end): Constify.
	* linespec.c (linespec_lex_to_end)
	(decode_line_with_current_source)
	(decode_line_with_last_displayed): Constify.
	* guile/scm-breakpoint.c (gdbscm_register_breakpoint_x):
	Constify.
	* cli/cli-cmds.c (edit_command, list_command): Constify.
	* breakpoint.h (until_break_command, watch_command_wrapper)
	(awatch_command_wrapper, rwatch_command_wrapper)
	(init_ada_exception_breakpoint): Constify.
	* breakpoint.c (break_command_1, dprintf_command)
	(break_range_command, watch_command_wrapper)
	(rwatch_command_wrapper, awatch_command_wrapper)
	(until_break_command, init_ada_exception_breakpoint)
	(strace_marker_create_sals_from_location, trace_command)
	(ftrace_command, strace_command, struct tracepoint): Constify.
	* ax-gdb.c (agent_command_1): Constify.
	* ada-lang.c (ada_exception_sal): Constify.

gdb/python/python.c

@@ -647,7 +647,7 @@ gdbpy_solib_name (PyObject *self, PyObject *args)
 static PyObject *
 gdbpy_decode_line (PyObject *self, PyObject *args)
 {
-  char *arg = NULL;
+  const char *arg = NULL;
   gdbpy_ref<> result;
   gdbpy_ref<> unparsed;
   event_location_up location;