Support an "unlimited" number of user-defined argumentsusers/palves/cxx-eliminate-cleanups-v2

I recently wrote a user-defined command that would benefit from
supporting an unlimited number of arguments:

 http://palves.net/list-active-signal-handlers-with-gdb/

However, we currently only support up to 10 arguments passed to
user-defined commands ($arg0..$arg9).

I can't find a good reason for that, other than "old code with hard
coded limits".  This patch removes that limit and modernizes the code
along the way:

  - Makes the user_args struct a real C++ class that uses std::vector
    for storage.

  - Removes the "next" pointer from within user_args and uses a
    std::vector to maintain a stack instead.

  - Adds a new RAII-based scoped_user_args_level class to help
    push/pop user args in the stack instead of using a cleanup.

I also needed a way to convert a number to a std::string, so I added a
new utility for that, gdb::to_string.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention that user commands now accept an unlimited number
	of arguments.
	* cli/cli-script.c: Include <vector>.
	(struct string_view): New type.
	(MAXUSERARGS): Delete.
	(struct user_args): Now a C++ class.
	(user_args_stack): New.
	(struct scoped_user_args_level): New type.
	(execute_user_command): Use scoped_user_args_level.  Adjust to
	call insert_user_args instead of insert_args.
	(arg_cleanup): Delete.
	(setup_user_args): Deleted, and refactored as ...
	(user_args::user_args): ... this new constructor.  Limit of number
	of arguments removed.
	(insert_args): Delete.
	(insert_user_args) New.
	(user_args::insert_args): New, bits based on old insert_args with
	limit of number of arguments eliminated.
	* common/common-utils.h: Include <sstream>.
	(gdb::to_string): New.

gdb/doc/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (User-defined Commands): Limit on number of
	arguments passed to user-defined commands removed; update.

1 files changed, 3 insertions, 0 deletions

diff --git a/gdb/NEWS b/gdb/NEWS
index 4a614388566..2317dd82099 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -17,6 +17,9 @@
 
 * Support for Java programs compiled with gcj has been removed.
 
+* User commands now accept an unlimited number of arguments.
+  Previously, only up to 10 was accepted.
+
 * New targets
 
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