Enhancement: Configurable progress reports for non-interactive terminals

The `backup`, `check` and `prune` commands never printed any progress
reports on non-interactive terminals. This behavior is now configurable
using the `RESTIC_PROGRESS_FPS` environment variable. Use for example a
value of `1` for an update every second, or `0.01666` for an update every
minute.

The `backup` command now also prints the current progress when restic
receives a `SIGUSR1` signal.

Setting the `RESTIC_PROGRESS_FPS` environment variable or sending a `SIGUSR1`
signal prints a status report even when `--quiet` was specified.

https://github.com/restic/restic/issues/2706
https://github.com/restic/restic/issues/3194
https://github.com/restic/restic/pull/3199