storagetm: expose more useful metadata for nvme block devices

don't let the devices to be announced just as model "Linux". Let's instead
propagate the underlying block device's model. Also do something
reasonably smart for the serial and firmware version fields.

docs/ENVIRONMENT.md

@@ -583,3 +583,14 @@ SYSTEMD_HOME_DEBUG_SUFFIX=foo \
 * `$SYSTEMD_FIREWALL_BACKEND` – takes a string, either `iptables` or
   `nftables`. Selects the firewall backend to use. If not specified tries to
   use `nftables` and falls back to `iptables` if that's not available.
+
+`systemd-storagetm`:
+
+* `$SYSTEMD_NVME_MODEL`, `$SYSTEMD_NVME_FIRMWARE`, `$SYSTEMD_NVME_SERIAL`,
+  `$SYSTEMD_NVME_UUID` – these take a model string, firmware version string,
+  serial number string, and UUID formatted as string. If specified these
+  override the defaults exposed on the NVME subsystem and namespace, which are
+  derived from the underlying block device and system identity. Do not set the
+  latter two via the environment variable unless `systemd-storagetm` is invoked
+  to expose a single device only, since those identifiers better should be kept
+  unique.