Allow the fallback hostname to be overriden using an environment variable

See https://bugzilla.redhat.com/show_bug.cgi?id=1893417 for the back story:
the fallback hostname matters a lot in certain environments. Right now the only
way to configure the fallback hostname is by recompiling systemd, which is
obviously problematic in case when the fallback hostname shall differ between
different editions of the same distro that share a single compiled rpm.

By making this overridable through an envvar, we're providing an escape hatch
without making this a top-level api. Later on a way to set this through
os-release is added, but I think the approach with the variable is still
useful. It it very convenient for testing, or to override settings only in a
particular service, etc.

docs/ENVIRONMENT.md

@@ -55,6 +55,10 @@ All tools:
 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
   SystemdOptions EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
 
+* `$SYSTEMD_DEFAULT_HOSTNAME` — override the compiled-in fallback hostname
+  (relevant in particular for the system manager and `systemd-hostnamed`).
+  Must be a valid hostname (either a single label or a FQDN).
+
 * `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
   method. Defaults to `auto`. Behavior is defined as follows:
   `auto`: Checks if `/etc/initrd-release` exists, and a temporary fs is mounted