Add command support for Guile.

Add command support for Guile.

	* Makefile.in (SUBDIR_GUILE_OBS): Add scm-cmd.o.
	(SUBDIR_GUILE_SRCS): Add scm-cmd.c.
	(scm-cmd.o): New rule.
	* guile/guile-internal.h (gdbscm_gc_xstrdup): Declare.
	(gdbscm_user_error_p): Declare.
	(gdbscm_parse_command_name): Declare.
	(gdbscm_valid_command_class_p): Declare.
	(gdbscm_initialize_commands): Declare.
	* guile/guile.c (initialize_gdb_module): Call
	gdbscm_initialize_commands.
	* guile/lib/gdb.scm: Export command symbols.
	* guile/lib/gdb/init.scm (%exception-keys): Add gdb:user-error.
	(throw-user-error): New function.
	* guile/scm-cmd.c: New file.
	* guile/scm-exception.c (user_error_symbol): New static global.
	(gdbscm_user_error_p): New function.
	(gdbscm_initialize_exceptions): Set user_error_symbol.
	* scm-utils.c (gdbscm_gc_xstrdup): New function.

	testsuite/
	* gdb.guile/scm-cmd.c: New file.
	* gdb.guile/scm-cmd.exp: New file.

	doc/
	* guile.texi (Guile API): Add entry for Commands In Guile.
	(Basic Guile) <parse-and-eval>: Add reference.
	(Basic Guile) <string->argv>: Move definition to Commands In Guile.
	(GDB Scheme Data Types): Mention <gdb:command> object.
	(Commands In Guile): New node.