Constify commands maint.c, plus maintenance_print_type

Constify commands maint.c, plus maintenance_print_type

In addition to the constification, this fixes a command-repeat bug.

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

	* typeprint.c (maintenance_print_type): Constify.
	* maint.c (maintenance_dump_me, maintenance_demangle)
	(maintenance_time_display, maintenance_info_sections)
	(maintenance_print_statistics, maintenance_deprecate)
	(maintenance_undeprecate): Constify.
	(maintenance_do_deprecate): Constify.  Use std::string.
	(maintenance_selftest): Constify.
	* gdbtypes.h (maintenance_print_type): Constify.