The output is now
```
-v, --verbose                    be verbose (specify multiple times or a level using --verbose=n, max level/times is 2)
```
instead of
```
-v, --verbose n                  be verbose (specify multiple times or a level using --verbose=n, max level/times is 2)
```

This turns snapshotFilterOptions from cmd into a restic.SnapshotFilter
type and makes restic.FindFilteredSnapshot and FindFilteredSnapshots
methods on that type. This fixes #4211 by ensuring that hosts and paths
are named struct fields instead of unnamed function arguments in long
lists of such.

Timestamp limits are also included in the new type. To avoid too much
pointer handling, the convention is that time zero means no limit.
That's January 1st, year 1, 00:00 UTC, which is so unlikely a date that
we can sacrifice it for simpler code.

Fixes #4239.

Since 0.15 (#4020), inodes are generated as hashes of names, xor'd with
the parent inode. That means that the inode of a/b/b is

	h(a/b/b) = h(a) ^ h(b) ^ h(b) = h(a).

I.e., the grandchild has the same inode as the grandparent. GNU find
trips over this because it thinks it has encountered a loop in the
filesystem, and fails to search a/b/b. This happens more generally when
the same name occurs an even number of times.

Fix this by multiplying the parent by a large prime, so the combining
operation is not longer symmetric in its arguments. This is what the FNV
hash does, which we used prior to 0.15. The hash is now

	h(a/b/b) = h(b) ^ p*(h(b) ^ p*h(a))

Note that we already ensure that h(x) is never zero.

Collisions can still occur, but they should be much less likely to occur
within a single path.

Fixes #4253.

github.com/Azure/azure-sdk-for-go/sdk/storage/azblob still uses v0.5.1
as upgrading it would increase the minimum Go version on Solaris to
1.20.

The snapshot filtering internally converts relative paths to absolute
ones to ensure that the parent snapshots selection works for backups of
relative paths.

Fixes #2260, #4191.