gdb: rework 'osv zfs' to provide better ARC data

New output:
(gdb) osv zfs
:: ZFS TUNABLES ::
	zil_replay_disable:       0
	zfs_nocacheflush:         0
	zfs_prefetch_disable:     0
	zfs_no_write_throttle:    0
	zfs_txg_timeout:          5
	zfs_write_limit_override: 0
	vdev_min_pending:         4
	vdev_max_pending:         10

:: ARC SIZES ::
	Current size:    1493512 (1 MB)
	Target size:     1341923840 (1279 MB)
	Min target size: 167740480 (159 MB)
	Max target size: 1341923840 (1279 MB)

:: ARC SIZE BREAKDOWN ::
	Most recently used cache size:   670961920 (639 MB) (50.00%)
	Most frequently used cache size: 670961920 (639 MB) (50.00%)

:: ARC EFFICIENCY ::
	Cache access total:  136422
	Cache hit ratio:     132326 (97.00%)
	Cache miss ratio:    4096 (3.00%)
	REAL hit ratio:      132751 (97.31%)

	Data demand efficiency:   99.99%
	Data prefetch efficiency: 0.00%

:: CACHE HITS BY CACHE LIST ::
	Most recently used:           108539 (82.02%)
	Most frequently used:         24212 (18.30%)
	Most recently used (Ghost):   0 (0.00%)
	Most frequently used (Ghost): 0 (0.00%)

:: CACHE HITS BY DATA TYPE ::
	Demand data:       127471 (96.33%)
	Prefetch data:     0 (0.00%)
	Demand metadata:   4855 (3.67%)
	Prefetch metadata: 0 (0.00%)

:: CACHE MISSES BY DATA TYPE ::
	Demand data:       8 (0.20%)
	Prefetch data:     3998 (97.61%)
	Demand metadata:   86 (2.10%)
	Prefetch metadata: 4 (0.10%)

:: ARC HASH INFO ::
	Total hash elements: 4103
	Max hash elements:   4103
	Hash collisions:     77
	Hash chains:         55

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>