Instead of filtering all lines for `PRETTY_LINE` and then replace the
line with only the quoted contents, `re-serach-forward` (or -backward)
provide a `match-*` interface. That way we can work solely inside the
buffer and return only the matching string part.
While `/etc/os-release` is usually available on Linux systems, it
might be missing on distribution that don't follow that convention.
For those distributions, the commands `hostnamectl` (from systemd) and
`lsb_release` might provide an alternative. Unfortunately, those
commands might also be missing as well as `/etc/os-release`, so the
newly introduced `eshell-info-banner--get-os-information` function
falls back to the string "Unknown" (and should never fail).
Fixes #1.
Benjamin Kästner