Don't throw Scheme exceptions with live std::vector objects

A complication with the Guile code is that we have two types of
exceptions to consider: GDB/C++ exceptions, and Guile/SJLJ exceptions.

Because Guile exceptions are SJLJ based, we must make sure to not have
live local variables of types with non-trivial dtors when a Guile
exception is thrown, because the dtors won't be run when a Guile
exceptions is thrown.

gdbscm_parse_function_args currently violates this:

 void
 gdbscm_parse_function_args (const char *func_name,
			     int beginning_arg_pos,
			     const SCM *keywords,
			     const char *format, ...)
 {
 ...
   /* Keep track of malloc'd strings.  We need to free them upon error.  */
   std::vector<char *> allocated_strings;
 ...
   for (char *ptr : allocated_strings)
     xfree (ptr);
   gdbscm_throw (status); /// dtor of "allocated_strings" is not run!
 }

This commit fixes the above making using of gdbscm_wrap.

It would be nice if we had a way to make it impossible to write such
code.  PR guile/23429 has an idea for that, if someone's interested.

gdb/ChangeLog:
2018-08-21  Pedro Alves  <palves@redhat.com>

	* guile/scm-utils.c (gdbscm_parse_function_args_1): New, factored
	out from gdbscm_parse_function_args.
	(gdbscm_parse_function_args): Rework to use gdbscm_wrap and
	gdbscm_parse_function_args_1.

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