Make extract_arg return a std::string

Change extract_arg to return a std::string and fix up all the users.
I think string is mildly better than unique_xmalloc_ptr<char>, when
possible, because it provides a more robust API.

I changed the error messages emitted from find_location_by_number to
avoid either writing to a string or an extra allocation; this can be
changed but I thought that the new message was not any less clear.
You can see an example in the testsuite patch.

ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* demangle.c (demangle_command): Update.
	* breakpoint.c (disable_command): Update.
	(enable_command): Update.
	(find_location_by_number): Make "number" const.  Use
	get_number_trailer.
	* cli/cli-utils.c (extract_arg): Return std::string.
	* probe.c (parse_probe_linespec): Update.  Change types.
	(collect_probes): Take string arguments.
	(parse_probe_linespec): Likewise.
	(info_probes_for_ops): Update.
	(enable_probes_command): Update.
	(disable_probes_command): Update.
	* break-catch-sig.c (catch_signal_split_args): Update.
	* mi/mi-parse.c (mi_parse): Update.

testsuite/ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* gdb.base/ena-dis-br.exp (test_ena_dis_br): Update test.

gdb/cli/cli-utils.c

@@ -249,36 +249,36 @@ remove_trailing_whitespace (const char *start, const char *s)
 
 /* See documentation in cli-utils.h.  */
 
-char *
+std::string
 extract_arg (const char **arg)
 {
   const char *result;
 
   if (!*arg)
-    return NULL;
+    return std::string ();
 
   /* Find the start of the argument.  */
   *arg = skip_spaces (*arg);
   if (!**arg)
-    return NULL;
+    return std::string ();
   result = *arg;
 
   /* Find the end of the argument.  */
   *arg = skip_to_space (*arg + 1);
 
   if (result == *arg)
-    return NULL;
+    return std::string ();
 
-  return savestring (result, *arg - result);
+  return std::string (result, *arg - result);
 }
 
 /* See documentation in cli-utils.h.  */
 
-char *
+std::string
 extract_arg (char **arg)
 {
   const char *arg_const = *arg;
-  char *result;
+  std::string result;
 
   result = extract_arg (&arg_const);
   *arg += arg_const - *arg;