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.
Previously the global context was either accessed via gopts.ctx,
stored in a local variable and then used within that function or
sometimes both. This makes it very hard to follow which ctx or a wrapped
version of it reaches which method.
Thus just drop the context from the globalOptions struct and pass it
explicitly to every command line handler method.
These commands filter the snapshots according to some criteria which
essentially requires loading the index before filtering the snapshots.
Thus create a copy of the snapshots list beforehand and use it later on.
The `dump`, `find`, `forget`, `ls`, `mount`, `restore`, `snapshots`,
`stats` and `tag` commands will now take into account multiple
`--host` and `-H` flags.