Enhancement: Support specifying multiple host flags for various commands

Previously commands didn't take more than one `--host` or `-H` argument into account, which could be limiting with e.g.
the `forget` command.

The `dump`, `find`, `forget`, `ls`, `mount`, `restore`, `snapshots`, `stats` and `tag` commands will now take into account
multiple `--host` and `-H` flags.

https://github.com/restic/restic/issues/1570
