gdb: improve 'osv zfs' command

This patch improves the command by adding useful info for debugging ZFS
in general, and also addresses some stylistic issues.

The new output is as follow:
(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 ::
	Actual ARC Size:        122905056
	Target size of ARC:     1341923840
	Min Target size of ARC: 167740480
	Max Target size of ARC: 1341923840
		Most Recently Used (MRU) size:   670961920 (50.00%)
		Most Frequently Used (MFU) size: 670961920 (50.00%)
:: ARC EFFICIENCY ::
Total ARC accesses: 42662
	ARC hits: 41615 (97.55%)
		ARC MRU hits: 12550 (30.16%)
			Ghost Hits: 0
		ARC MFU hits: 29045 (69.79%)
			Ghost Hits: 0
	ARC misses: 1047 (2.45%)
Prefetch workload ratio: 0.0097%
Prefetch total:          412
	Prefetch hits:   20
	Prefetch misses: 392
Total Hash elements: 1053
	Max Hash elements: 1053
	Hash collisions:   13
	Hash chains:       11

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