sd-journal: allow to specify compression algorithm through env

Fixes RHBZ#2183546 (https://bugzilla.redhat.com/show_bug.cgi?id=2183546).

Previously, journal file is always compressed with the default algorithm
set at compile time. So, if a newer algorithm is used, journal files
cannot be read by older version of journalctl that does not support the
algorithm.

Co-authored-by: Colin Walters <walters@verbum.org>

docs/ENVIRONMENT.md

@@ -513,6 +513,14 @@ SYSTEMD_HOME_DEBUG_SUFFIX=foo \
   journal. Note that journal files in compact mode are limited to 4G to allow use of
   32-bit offsets. Enabled by default.
 
+* `$SYSTEMD_JOURNAL_COMPRESS` – Takes a boolean, or one of the compression
+  algorithms "XZ", "LZ4", and "ZSTD". If enabled, the default compression
+  algorithm set at compile time will be used when opening a new journal file.
+  If disabled, the journal file compression will be disabled. Note that the
+  compression mode of existing journal files are not changed. To make the
+  specified algorithm takes an effect immediately, you need to explicitly run
+  `journalctl --rotate`.
+
 `systemd-pcrphase`, `systemd-cryptsetup`:
 
 * `$SYSTEMD_FORCE_MEASURE=1` — If set, force measuring of resources (which are