Add to neotest bench-disk command that performs random-read disk
benchmarks via ioping. Example output:

	(venv) (8) neotest@rio:~/8/src/lab.nexedi.com/kirr/neo/go/neo/t$ ./neotest bench-disk
	node:   rio
	cluster:

	*** disk: random direct (no kernel cache) 4K-read latency

	--- . (ext4 /dev/sda1) ioping statistics ---
	29.1 k requests completed in 2.95 s, 113.6 MiB read, 9.85 k iops, 38.5 MiB/s
	generated 29.1 k requests in 3.00 s, 113.6 MiB, 9.69 k iops, 37.9 MiB/s
	min/avg/max/mdev = 43.2 us / 101.5 us / 250.0 us / 7.48 us
	Benchmarkdisk/randread/direct/4K-min 1 43.2 us/op
	Benchmarkdisk/randread/direct/4K-avg 1 101.5 us/op
	< 59.2 us       458     |
	< 63.0 us       0       |
	< 66.7 us       0       |
	< 70.5 us       0       |
	< 74.2 us       1       |
	< 78.0 us       1       |
	< 81.7 us       0       |
	< 85.5 us       1       |
	< 89.2 us       0       |
	< 93.0 us       0       |
	< 96.7 us       0       |
	< 100.5 us      333     |
	< 104.2 us      27793   | ***********************************************
	< 108.0 us      259     |
	< 111.7 us      21      |
	< 115.5 us      8       |
	< 119.2 us      18      |
	< 123.0 us      1       |
	< 126.7 us      0       |
	< 130.5 us      7       |
	< 134.2 us      59      |
	<       +∞      18      |
	# POLL·186 C1·291360 C1E·290802 C3·31 C6·1218

	...

	*** disk: random cached 4K-read latency

	--- . (ext4 /dev/sda1) ioping statistics ---
	3.15 M requests completed in 2.82 s, 12.0 GiB read, 1.12 M iops, 4.26 GiB/s
	generated 3.15 M requests in 3.00 s, 12.0 GiB, 1.05 M iops, 4.00 GiB/s
	min/avg/max/mdev = 465 ns / 896 ns / 37.4 us / 183 ns
	Benchmarkdisk/randread/pagecache/4K-min 1 465 ns/op
	Benchmarkdisk/randread/pagecache/4K-avg 1 896 ns/op
	< 839 ns        771375  | ************
	< 872 ns        609361  | *********
	< 905 ns        660635  | **********
	< 938 ns        505305  | ********
	< 971 ns        189182  | ***
	< 1.00 us       93655   | *
	< 1.04 us       70811   | *
	< 1.07 us       57650   |
	< 1.10 us       51587   |
	< 1.14 us       44648   |
	< 1.17 us       40868   |
	< 1.20 us       27301   |
	< 1.24 us       12503   |
	< 1.27 us       5580    |
	< 1.30 us       2517    |
	< 1.34 us       1404    |
	< 1.37 us       698     |
	< 1.40 us       378     |
	< 1.44 us       208     |
	< 1.47 us       119     |
	< 1.50 us       50      |
	<       +∞      978     |
	# POLL·1 C1·57 C1E·11 C3·2 C6·257

The benchmarks are so done so that output conforms to Go benchmarking
format. This way it is possible to process the output with benchstat to
summarize / compare results. For above run summarization gives:

	name                            time/op
	disk/randread/direct/4K-min     39.0µs ±12%
	disk/randread/direct/4K-avg      117µs ± 3%
	disk/randread/pagecache/4K-min   461ns ± 3%
	disk/randread/pagecache/4K-avg   880ns ± 0%

While at disk topic, teach info/info-local to show related information
about node's disk. Example line output for rio:

	disk/sda: Samsung SSD 840   rev BB0Q 931.5G

Please see

	http://navytux.spb.ru/~kirr/neo.html#the-need-for-faster-storage
	http://navytux.spb.ru/~kirr/neo.html#appendix-i-ssd-latency

for some discussion about SSD performance.

Some draft history related to this patch:

	lab.nexedi.com/kirr/neo/commit/d35a2fdf	X neotest/info-local: Fix disk display in presence of bind-mounts
	lab.nexedi.com/kirr/neo/commit/44529dbf	X neotest/bench-disk: Deduplicate code; change 1M -> 2M
	lab.nexedi.com/kirr/neo/commit/8bac3dba	X neotest/bench-disk: Also benchmark randomly reading 1M blocks
	lab.nexedi.com/kirr/neo/commit/e795c6ed	X neotest: Fix disk display in case of DM
	lab.nexedi.com/kirr/neo/commit/352cd100	X neotest: Fix disk display in case of MD
	lab.nexedi.com/kirr/neo/commit/cd2cd093	X bench_disk: Add std bench format
	lab.nexedi.com/kirr/neo/commit/9f86eb40	X bench += ioping