Make enable of disk cache statistics code configurable.

* configure.ac: --enable-cache-stats added.
* config.h.in (DISK_CACHE_STATS): New define.
* grub-core/Makefile.core.def (cacheinfo): New command.
* include/grub/disk.h(grub_disk_cache_get_performance): New function.
* grub-core/commands/cacheinfo.c: New file.
* grub-core/commands/minicmd.c (grub_rescue_cmd_info): Updated and
moved to cacheinfo.c.
* grub-core/kern/disk.c: Use DISK_CACHE_STATS to disable disk cache
debug code.
* include/grub/disk.h: Likewise.